You might try holding the power button on the PC down for about 10 seconds or so to force the hard boot. Simply depressing it will usually not let it shut down when it is in that state.
If it is a desktop PC, remove the power plug from the power outlet for a few minutes if the above doesn't work. If it is a laptop, remove it from AC power *and* remove and replace the battery. That will shut it down hard.
-steve