Monitor goes to sleep

i boot my computer windows xp it boots up normally goes to the bar loading screen then after the monitor goes to sleep... any ideas why? it wasnt having this problem before... also side note an hour prior i had tried hooking it up to my hdtv but there was no signal so i plugged my monitor in to change the settings within the nvidia graphics settings but i couldnt reach that far because the monitor falls alseep after windows xp loading screen