Turn your PC off and disconnect the power cord. open up the case and find the CMOS battery on the motherboard. It'll most likely be a round Lithium cell in a clip holder. Remove the battery.
After waiting for a while to allow any residual charge to drain (longer is better, but several minutes could be enough) replace the power and try starting up again.
It sounds like the 'hard' power off might have corrupted the CMOS settings, and removing/replacing the battery should reset BIOS to defaults again for you.
Catweazle
Grandad
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