computer turns off

I have a Dell Inspiron 660 s and have added a monitor "Dell Monitor for both, both the same" after about 2hrs some times longer the computer will just turn off ! one of the monitors goes through my VGA port and the other through the HDMI port these slots came with the computer from new about 5 months ago and have the same same graphic card no changes within the computer from new at all, can you tell me why this would happen please. regards Jim7777