If the power fails cleanly, then it should recover itself, and boot OK. If in doubt, turn off the switches on the power supplies and wait for 30 seconds, and power them back on again. Or wait until the power supply to the building is restored, and stable, then power on the switches on the power supplies. It should boot back and work OK, this takes a few minutes, but it does get there in the end.
If it fails to boot correctly, connect a PC serial port to the Console port on the supervisor, and use Hyperterminal (or better is TeraTerm) set to 9600, 8 bit, no parity, to record the sequence, and post it here. We can then help you to resolve the problem.