Game Freezes

Hey All,
THis is the first time I ever had a problem with my system. In the middle of the game my screen freezes and then the screen becomes black. The only way to get out of the game is to manually reset the computer. Any idea what it could be? My video card is not overclocked.

Thanks

could be its getting a tad hot try taking the sink off and reapllying some AS5 or what ever you use or the psu is going out