Tried the BIOS and starts were never quite the same each time.
If your BIOS starts are failing then it has nothing to do with Windows. If your PSU failed perhaps it damaged something sensitve in the laptop on its way out.
If I've read the OP wrong and it is windows, then in one past experience due to a power failure I lost some Windows system files and got some weird behaviour. The fix was easy: Boot of an alternate source (I had dual boot) and restore the Windows folder from a backup.