Narrative:

Took off from runway xxr; captain was pilot flying. My FMS was on the legs page; and captain was on the takeoff reference page. Passing 10;000 [feet] while climbing to FL150 on the SID; the captain asked me to park the ILS so we don't forget to do it later. I reached down and pressed the navigation/rad key on my FMS and it didn't switch to the navigation/rad page. I pressed it a couple more times with no joy. At that time; I mentioned it to the captain that it seemed like my FMS wasn't responding and it appeared to be frozen; so I was going to try his side. I then pressed the navigation/rad key on his side with no change in page; his side was still showing the takeoff ref page. At that point he engaged the auto pilot. He attempted pressing the navigation/rad key as well as some other random keys with no change to the FMS display. Shortly after; ATC assigned us a new altitude of FL250; we were approaching the previously assigned altitude of FL150; so the FMA had already switched from VNAV speed to VNAV altitude because FL150 was a restriction on the departure. I asked ATC if we could get unrestricted climb to FL250 which they approved; at which time the captain pressed the altitude selector to release the altitude but it had no effect; was still showing VNAV altitude (or altitude hold) and the FL150 altitude constraint was still showing in the FMS. The captain then selected flch climb; we notified ATC that we had a malfunction with our navigation and needed delay vectors while we ran checklist and tried to correct the issue. At that point they gave us an initial heading slightly east of the departure route. Captain asked me to grab the QRH and run the checklist; since we didn't have any EICAS messages; I looked all over the QRH and didn't find any checklist that applied to our malfunction. I asked the captain to look also in case I missed something. At that point; he transferred the controls to me and looked in the QRH also but couldn't find any applicable checklist. Both of our FMS/mcdu were still locked up at this point; and we were getting closer to the [another country's airspace] so he asked for delay vectors back to ZZZZ for landing. I took over the radios and still had the airplane; the captain then started messaging [dispatch] to communicate our situation via ACARS. Then; [we] switched to sat/com since it was more efficient but was somewhat intermittent. ZZZZ control kept inquiring fairly often if we were ready to receive vectors for approach and landing; but we kindly let them know that we would advise when ready. They then asked us to descend to FL160. By this point; we were on a vector to the south/southeast heading in the direction of the airport; and ZZZZ control asked again if we were ready for vectors to landing; I then suggested to the captain that maybe we ask to do some right hand orbits from our current position to give us a little more time and free up ATC from having to vector us; they approved. They seemed a little busy with other arrivals also; so I figured the orbits would help. Shortly after; the captain was able to coordinate with [dispatch]/maintenance over the satcom and they recommended we reset some breakers to try to restore our FMS/mcdu's. Due to the unknown result of resetting the breakers; the amount of time required to reset everything; and not knowing the reliability of the navigation even after we reset it; we decided it was best to [request priority] at this time which we did. The captain got up from the seat to complete the FMC reset while I flew the airplane and worked the radios. After about 9 minutes of right hand orbits; we had both left/right FMS/mcdu's up and running properly; but not knowing the reliability of the system or what caused the failure in the first place; or if there were any other issues we weren't aware off; we decided the best option was to land in ZZZZ. At that point; we were pretty close to the airport and the weather was VFR; and maintenance could get a chance to look over the airplane before we took it into [another country's] airspace. Wegathered all the required information for landing; and requested vectors to runway xxl. We landed safely; no assistance needed on the ground.in hindsight; I should have probably requested VFR hold with 3-5 minute legs; but initially; I thought just a couple of turns and we would be on our way for approach and landing; didn't think I'd still be turning the heading knob 9-10 minutes later. Hindsight is 20/20.FMS mcdu's frozen/locked up on both captain and first officer side; cause for the malfunction is unknown.[suggestion is that the] only way would be to have fail proof equipment.

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Original NASA ASRS Text

Title: Air carrier flight crew was in climbout from departure airport. During the climb neither pilot was able to use either FMC as they had froze. After troubleshooting the crew decided to return to their departure airport.

Narrative: Took off from Runway XXR; Captain was Pilot Flying. My FMS was on the LEGS page; and Captain was on the takeoff REF page. Passing 10;000 [feet] while climbing to FL150 on the SID; the Captain asked me to PARK the ILS so we don't forget to do it later. I reached down and pressed the NAV/RAD key on my FMS and it didn't switch to the NAV/RAD page. I pressed it a couple more times with no joy. At that time; I mentioned it to the Captain that it seemed like my FMS wasn't responding and it appeared to be frozen; so I was going to try his side. I then pressed the NAV/RAD key on his side with no change in page; his side was still showing the takeoff ref page. At that point he engaged the auto pilot. He attempted pressing the NAV/RAD key as well as some other random keys with no change to the FMS display. Shortly after; ATC assigned us a new altitude of FL250; we were approaching the previously assigned altitude of FL150; so the FMA had already switched from VNAV SPD to VNAV ALT because FL150 was a restriction on the departure. I asked ATC if we could get unrestricted climb to FL250 which they approved; at which time the captain pressed the ALT selector to release the altitude but it had no effect; was still showing VNAV ALT (or ALT HOLD) and the FL150 altitude constraint was still showing in the FMS. The Captain then selected FLCH climb; we notified ATC that we had a malfunction with our navigation and needed delay vectors while we ran checklist and tried to correct the issue. At that point they gave us an initial heading slightly east of the departure route. Captain asked me to grab the QRH and run the checklist; since we didn't have any EICAS messages; I looked all over the QRH and didn't find any checklist that applied to our malfunction. I asked the Captain to look also in case I missed something. At that point; he transferred the controls to me and looked in the QRH also but couldn't find any applicable checklist. Both of our FMS/MCDU were still locked up at this point; and we were getting closer to the [another country's airspace] so he asked for delay vectors back to ZZZZ for landing. I took over the radios and still had the airplane; the Captain then started messaging [Dispatch] to communicate our situation via ACARS. Then; [we] switched to SAT/com since it was more efficient but was somewhat intermittent. ZZZZ Control kept inquiring fairly often if we were ready to receive vectors for approach and landing; but we kindly let them know that we would advise when ready. They then asked us to descend to FL160. By this point; we were on a vector to the South/SE heading in the direction of the airport; and ZZZZ Control asked again if we were ready for vectors to landing; I then suggested to the Captain that maybe we ask to do some right hand orbits from our current position to give us a little more time and free up ATC from having to vector us; they approved. They seemed a little busy with other arrivals also; so I figured the orbits would help. Shortly after; the Captain was able to coordinate with [Dispatch]/Maintenance over the SATCOM and they recommended we reset some breakers to try to restore our FMS/MCDU's. Due to the unknown result of resetting the breakers; the amount of time required to reset everything; and not knowing the reliability of the NAV even after we reset it; we decided it was best to [request priority] at this time which we did. The Captain got up from the seat to complete the FMC reset while I flew the airplane and worked the radios. After about 9 minutes of right hand orbits; we had both L/R FMS/MCDU's up and running properly; but not knowing the reliability of the system or what caused the failure in the first place; or if there were any other issues we weren't aware off; we decided the best option was to land in ZZZZ. At that point; we were pretty close to the airport and the weather was VFR; and Maintenance could get a chance to look over the airplane before we took it into [another country's] airspace. Wegathered all the required information for landing; and requested vectors to Runway XXL. We landed safely; no assistance needed on the ground.In hindsight; I should have probably requested VFR hold with 3-5 minute legs; but initially; I thought just a couple of turns and we would be on our way for approach and landing; didn't think I'd still be turning the heading knob 9-10 minutes later. Hindsight is 20/20.FMS MCDU's frozen/locked up on both Captain and First Officer side; cause for the malfunction is unknown.[Suggestion is that the] only way would be to have fail proof equipment.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.