In addition to the other answers, large generators usually use externally-energised coils to provide their magnetic field, so they can't generate unless there is already a supply of electricity. Then there's all the peripheral machinery of a power plant such as coal crushers and feeders, pumps, valves and so on.

Somewhere there will be a manual for the unfortunate operators who will have to restart the whole grid in the correct order, relying on the telephone company's backup generators to communicate.