Screen goes black automatically!!!!

Hello,
after doing some work on the computer ,the screen goes black,i can hear the sound of fan running and the lights of CPU also glow...but i cant do anything...pendrive stops to detect....keyboard doesnt works(unable to turn keyboard lights on/off) ,I have even tried to change the slot of ram but it doesnt worked!!computer only works for 10-15 minutes...if a heavy load work(like playing game)is done then suddenly the screen goes black!!....please help
my motherboard is 945 S chipset dual core 1.8 Ghz 1gb ram
kindly Help!!!!