Failing to notice OFF Engine Generator Control switches prior to engine start; a B757 Flight Crew suffered a complete loss of AC electrical power upon APU shut down. Additional anomalies ensued enroute;likely the result of the interrupted power and the decision not to shut down and restart the prestart set-ups and flows.

Narrative

On taxi-out (1st flight of the day on an aircraft that we'd flown in the previous night) we experienced a complete AC power loss after engine start because we had not noticed that someone had selected both engine generator control switches to OFF. It is not a very obvious condition and we had both missed it on preflight. So; when we shut down the APU after engine start; we experienced a complete power loss until my sharp-eyed Copilot discovered the reason. We re-engaged the switches; restoring power; and; of course; it took a couple of minutes for many electronics to come back up to speed. However; we did have to re-load the items that 'dumped' from the FMC. We then performed the appropriate checklists; and all systems seemed to be normal at that point. During the takeoff and the initial climb; we noted several display anomalies (for example; partial EPR information missing) as well as airspeed bug and takeoff mode FMC anomalies. All were noted after the takeoff was committed and none were serious enough to abort for. Undoubtedly they were related to the ground power loss. But while we were trying to figure out just exactly what was working and what wasn't we both missed the fact that our heading bug was not set on 350 degrees as required for that portion of the departure; but on 300 degrees instead. The First Officer was flying and he rolled past 350 degrees and was almost at 310 degrees when I noticed it and directed him to come back to the right. I reset the bug to 350...just then Departure Control transferred us to Center. When we checked in with Center; I asked if a heading of 350 was still required and Center replied yes; and then shortly thereafter issued us a direct-to clearance. At no time did Departure Control or Center indicate that they were concerned about our position or heading. A classic case of being too distracted with a non-normal situation. At some point during the climb-out we also noted a 'STATUS' indication on the lower EICAS screen; on calling up the EICAS status page; L & R oil cooling messages were displayed. They were also probably related to the power loss and should have been addressed prior to takeoff if it was present. We tried clearing them in flight; and they would not clear (until after we shut down the engines after arrival; when they cleared easily; confirming that they were spurious messages and not a malfunction). So; in summary; we had missed switch positions on the overhead panel during preflight; possibly missed a 'status message' indication during preflight (and subsequently a logbook entry and dispatch inquiry missed) and a minor EPR gage anomaly; and missed a mandatory heading after takeoff. I think the preflight error was due to the obscurity of the switch position; the rarity of the use of those switches; and perhaps a little complacency since we were the ones who brought the aircraft in the day prior and assumed that no-one would tamper with important controls. Our failure to notice a status message; if indeed we missed it; and partial EPR gage information loss was due to being confronted with an unfamiliar problem that affects so many systems in insidious ways; and also pressure to takeoff before we burned beyond our minimum takeoff fuel (which was rapidly approaching). If I had to do it all over again; I would just slow everything down; even though an expensive delay might have ensued. To the best of my knowledge; my company has not taken issue with any of our actions; but I am certainly smarter for the experience.