NVIDIAs alternate drivers for OS X usually break after a minor system update. You can fix this most of the time by modifying the following file:
Change the NVDIARequiredOS to your current system version which can be found in the app System Information. The version number should look something like this:
Write it into the file and save it (sudo access is needed).
You should now rebuild your kext cache:
sudo kextcache -system-prelinked-kernel sudo kextcache -system-caches
Reboot. Select the NVIDIA web drivers in your system settings and reboot once more. Done.