Hold the power button for about 5 second and the computer should shut down completely.
If your still not getting anything:
1- for a tower - unplug the tower completely and wait for a minutes and then try again.
If nothing happens, probably have 1-defective psu or motherboard
2- for laptop, unplug and get the battery out of the lappy,
hold the power button for 30 second
then replug the lappy, if it starts you can put the battery back.
if not you might have a dead lappy.
Then disable and don't use sleep or hibernation because it causes problems.