There should not have been any way for the utility power to reach your generator, unless you were backfeeding it into your main panel in a dangerous and illegal fashion. YOU MUST USE EITHER AN INTERLOCK OR A TRANSFER SWITCH. Otherwise you are likely backfeeding power to the grid via the transformer and could easily kill or injure a lineman. The transformer can step the generator power back up to thousands of volts on a line that's supposed to be dead. Chalk up your situation as an expensive learning experience that at least did not lead to manslaughter jail time.
Regardless of the generator being off, if both the main and generator breaker were on at the same time as the grid power was restored you likely fried the alternator on the generator.
Heres a fun fact, when the grid came back it juiced the generator energized the alternator spinning it backwards for who knows how long until some had to give.
Sorry, but it sounds like you're still not understanding it. If you're not using a Transfer Switch (automatic or manual), then your main and generator breakers MUST be interlocked so that they both cannot be on at the same time (for exactly this situation). If your "licensed union electrician" didn't install the interlock, he owes you a generator.Tabora, yes I’m very aware of this as my licensed union electrician installed and made me aware of this , I would turn the main off before use. I guess I didn’t mention the generator was off all night before electric cam on in the morning.