Probably Windows disabled your GPU.

Try disabling ULPS via the registry:
Go to:
[HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class{4D36E968-E325-11CE-BFC1-08002BE10318}]
Change "EnableULPS" to 0 under the keys that apply to your video card(s).
You should be able to tell which keys (0000, 0001, etc) apply to your video cards by looking through the keys for the DriverDesc or other information.