Try removing the mains power from the Power Supply and
plugging it back in.
If your BIOS is using soft power-off (meaning you have to
hold the power button in for several seconds to turn it
off), you might also like to try holding it in for several
seconds, and then pressing it again to turn it on... I
recently experienced a BIOS-related problem on one machine
and found that worked... the BIOS seemed to think the PC
was in a powered-on state (even tho it wasn't) and
wouldn't respond to me pressing the power button... until
I held it down for a while and the BIOS decided the PC was
now off.