Did you have a Windows update between working and not working?
What is your nVidia driver?
Right click in any open area of the Windows desktop
Select NVIDIA Control Panel from the popup window
Select SYSTEM INFORMATION at the lower left corner
Driver version should be the 1st line in Details
I have a GTX285 using driver 296.10 and Win7
Last update was the 12th of september so no updates since.
Driver is 320.18
If it makes any difference everything is working fine in after effects
When I import new clips their video wont playback either
restarting the computer, opening a new project, importing some random clips and then opening the project I wanted to work on seems to have fixed the issue.