If, for some reason, your NVIDIA GPU-powered graphics card decides that it doesn't want to kick into 3D mode -- that is, switch from low clock speeds to high ones for heavier demands such as gaming -- the usual fix is to reboot your machine. Sometimes this can be inconvenient to do, fortunately, there is a way to get your card running normally without slamming the power button.
On machines running Windows Vista or later, reinstalling your NVIDIA card's latest drivers will convince it to return to its proper running state. There's no need to select the "Perform a clean installation option", just locate or download the driver version you want and install it right over the top.
If you want to find uncompressed versions of recent NVIDIA drivers you've used, you can usually find them in the folder
C:\NVIDIA\DisplayDriver, if you follow the standard installer options.