What kind of monitor do you have? I have a sceptre monitor and when i had my x800gt in it it would not receive a signal form the video card after going to sleep and back on. After replacing it with an nividia or using the onboard video only did it work.
If reinstalling the drivers doesn't help try connecting the monitor through VGA input instead of DVI, if you can. Most cards come with a little DVI-to-VGA adapter for the cable. I ran into the same type of problem a few days ago, no signal when booting into Windows or after changing the resolution in Safe Mode. Turns out the monitor somehow corrupted itself and was sending faulty device ID information to the video card, now DVI input is just broken while VGA still works fine. Real bitch of a problem to fix because it involves re-flashing the EDID information in the monitor
It's not a common problem, but it's happened to quite a few people.