As they age, it happens.
But let it cool down and try again. Then if not, power down and remove the power cord. Gently push all cards home and try again.
No? Try System Restore to yesterday.
No? Then it's the usual failure I've seen too many of.
Yesterday, I was playing a game when all of a sudden the computer just shut down, I restarted and when I did all the icons and windows were extra large, managed to fix that but went to play again and now the mouse is sluggish and the graphics are taking forever. I went to the Device Manager and it has the yellow ! in, Nvdia Ge Force 8800 GTS 512