Res is stuck at 640 x 480?

I tried over clocking my 5770 with ati overdrive, went to far, and now my res is 640 x 900 and I cant change it back. I tried going into catalyst control center and other ways of changing the res back, but none of the windows fit in my screen so I cant click the apply on and stuff like that. Im really effed, some help would be nice.