Window 7 stops most functions but Running??

Hello, I have some p.c. experience but mainly with specialised software I used. The problem that I have is that windows]7 seems to stop (as if to reboot), the monitor goes blank but the fans are still working and there is power to the DVD drives. No amount of key pressing or mouse jiggling will coax it back to life and I have to do a hard reboot.

I have some times done this after several hours and the only errors I seem be able to find are for the hard reboot but there are info files up until that point which suggests the processors are working??

I have tried lots of things and I am getting frustrated please help