How do you hard reset an iMac that's completely frozen?

Try Cmd+Option+Esc first. If it's only one app that's hung, that brings up Force Quit and you keep everything else. If the power button doesn't do anything either, pull the power cable at the back, give it 15 seconds, plug back in.

hold the power button on the back (lower left if you reach round from the front) for about 10 seconds until it switches off, wait a bit, press it again. it's fine for the hardware, you just lose whatever wasn't saved. if it keeps freezing after that, that's the bit worth chasing.