I have a windows 7 x64 machine which sometimes (not all the time - no discernable pattern) when I tell it to sleep turns off the monitor but never powers off the machine (left it overnight). It then won't wake up, tried the keyboard, mouse and power button.

EDIT: 

to be clear the machine turns the screen off like it is going to sleep, but then the fans/disks/cpu do not turn off and the power light on the front of the machine stays on. Once it is in this state it can then be 'woken'

/EDIT

My only option seems to be to hard shut off and reboot.

Why might this be?