http://forums.nvidia.com/index.php?showtopic=178665
If anyone finds this of interest
it's a driver issue. when this happens you need to press F8 right before windows (win7 64) loading screen and select the option to start windows in low resolution (640x480) mode. then simply set your video options in nvidia control panel again. re-installing drivers might not help. this bug rears it's ugly head during a card swap or even when connecting different monitors / swapping out monitors or may happen at random.
symptoms are a black screen or stuck at windows logo, both during boot ups.
If anyone finds this of interest