Narrative:

We were at FL390 overflying the ZZZ area from ZZZ1 to ZZZ2 when we noticed a flicker of the lights followed by the EICAS messages indicating the left generator had gone off line. This was confirmed by the off light on the left gen control switch. The plane was on autopilot and held it; as we went into the QRH to rectify the issue. The first step of the QRH gave us one attempt to reset the generator; which we did. Upon that attempted reset; we instead lost the entire left AC bus and; in turn; all components associated with it. As pilot flying; and to make sure our first priority was safely flying the aircraft; I began to hand fly as the autopilot was kicked off and to make sure we were in full control. We completed the left gen off QRH which led us to starting the APU to acquire the extra generator and then went to get the left AC bus off QRH as that was our new issue. The first step in that procedure is again to try and reset the generator. That is only allowed once; we had already done it once; not to mention it had made the situation worse; so we couldn't perform that step again. It was at that time that the cockpit was getting very warm and I noticed my ADI lost all color including the artificial horizon. We noticed the equipment cooling overheat light had illuminated so we switched to alternate and regained full imaging on the ADI. The captain also switched his flight instruments to the first officer's (first officer) side as his had failed with the left AC bus. As we returned to the QRH to continue the procedure; we noticed our screens were getting dim and slightly flickering and since we were basically right over O'hare; we made the decision to get the plane down to the ground before things continued to get worse. The captain [requested priority handling] and we were vectored for a descent back to ZZZ. The captain informed the flight attendant (flight attendants) on what exactly was going on. We had no ACARS function so we could not contact anyone else but had ATC relay what information they could. The flight attendant's informed us that we had an first officer in the back who was willing to come up and help be an extra brain on the flight deck which was extremely helpful. She looked up our landing speeds and informed us of some of the components we would no longer have from the QRH; while the captain and I were getting the things we could finished with the QRH procedure and preparing for landing in ZZZ. Once established on the ILS to runway xxr; at about 4000 ft.; we transferred controls to the captain for the landing and the emergency trucks followed us to the gate.

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Original NASA ASRS Text

Title: B757 flight crew reported a diversion after losing the left AC Bus.

Narrative: We were at FL390 overflying the ZZZ area from ZZZ1 to ZZZ2 when we noticed a flicker of the lights followed by the EICAS messages indicating the left generator had gone off line. This was confirmed by the OFF light on the left gen control switch. The plane was on autopilot and held it; as we went into the QRH to rectify the issue. The first step of the QRH gave us one attempt to reset the generator; which we did. Upon that attempted reset; we instead lost the entire L AC bus and; in turn; all components associated with it. As Pilot Flying; and to make sure our first priority was safely flying the aircraft; I began to hand fly as the autopilot was kicked off and to make sure we were in full control. We completed the L GEN OFF QRH which led us to starting the APU to acquire the extra generator and then went to get the L AC BUS OFF QRH as that was our new issue. The first step in that procedure is again to try and reset the generator. That is only allowed once; we had already done it once; not to mention it had made the situation worse; so we couldn't perform that step again. It was at that time that the cockpit was getting very warm and I noticed my ADI lost all color including the artificial horizon. We noticed the equipment cooling overheat light had illuminated so we switched to alternate and regained full imaging on the ADI. The Captain also switched his flight instruments to the FO's (First Officer) side as his had failed with the L AC bus. As we returned to the QRH to continue the procedure; we noticed our screens were getting dim and slightly flickering and since we were basically right over O'Hare; we made the decision to get the plane down to the ground before things continued to get worse. The Captain [Requested Priority Handling] and we were vectored for a descent back to ZZZ. The Captain informed the FA (Flight Attendants) on what exactly was going on. We had no ACARS function so we could not contact anyone else but had ATC relay what information they could. The FA's informed us that we had an FO in the back who was willing to come up and help be an extra brain on the flight deck which was extremely helpful. She looked up our landing speeds and informed us of some of the components we would no longer have from the QRH; while the Captain and I were getting the things we could finished with the QRH procedure and preparing for landing in ZZZ. Once established on the ILS to Runway XXR; at about 4000 ft.; we transferred controls to the Captain for the landing and the emergency trucks followed us to the gate.

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.