I had a similar problem with my 5770 where it looked like I lost signal to my monitor. This was after attempting to OC it. However, when I logged in as administrator, I can see the gpu and the gpu actually worked. Had no probs running bc2 and other games as admin, but under my personal log on, I'd at least get the desktop for a few seconds before my monitor would lose signal. After installing and reinstalling drivers for a few hours, it just started to work again. Can you even get into safe mode or log in as admin?