Black screen when playing a game.

Hey guys I have seen a few post about stuff like this but they didn't have what I have going on. So a few weeks ago I'm playing World of Tanks and it was working fine for hours then I go to play Counter strike for a few hours nothing happen everything worked fine. Well when I got around to Dragon age I always fun in windows mode. I clicked the internet to look something up but then when I clicked back into Dragon age it gave me a black screen and a screen saver came up saying (Not Opitimum Mode- Recommended Mode 1280x1024 60Hz ?) I never seen this ever. So I took my friends monitor and hooked it up to mine and everything came up the game and all. But when I hook mine back up it goes back to this screen. People told me it's my video card but how could it be that if switching screens worked? Now I didn't use the screen long just a few minutes but I'm going crazy with out my GAMES :'(