What could cause my computer's display to go darker suddenly?

"My theory turned out to be correct: the NVidia drivers are to blame.

But, whatever the drivers may have done to mess things up, you can take hold of the drivers and force them back into submission. This is handy, since it eliminates the hassle of having to uninstall/reinstall the drivers from scratch.

After you've made-certain your monitors themselves are calibrated and functioning correctly, open the NVidia Control Panel.

There are several possible ways to do this:

Right-click on the desktop and select ""NVIDIA Control Panel""
OR
In the Display Properties box, find ""Settings,"" then ""Advanced,"" then the special NVidia tab for your video card, and then select ""Start the NVIDIA Control Panel""
OR
Right-click on the NVidia system-tray icon, and select ""NVidia Control Panel""

(There may be other ways to open the NVidia Control Panel, but I just name these few for now. If you can't find the NVidia Control Panel in any of these places, than you probably don't have the proper NVidia drivers installed correctly for your hardware.)

Once you have the NVidia Control Panel open, select ""Standard View"" from the drop-down near the main menu and the back/forward/home buttons. If ""Standard View"" is already selected, then you're good to go.

Under ""Select a Category,"" choose ""Display."" Then, under ""Appearance,"" choose ""Adjust desktop color settings."" Select your first monitor and, near the top-right of the window, select ""Restore Defaults."" Then select your next monitor and select ""Restore Defaults"" again. Repeat this process for all of your connected monitors.

Once the default settings have been restored, your NVidia video card will be in the default color mode for each display, but your monitors may still seem to be displaying incorrectly. To fix this, select each monitor and move the sliders below to adjust the display output. These sliders are very sensitive, and you shouldn't have to move them very much to get your display looking good again. With my CRT monitors, I found that I needed to increase the contrast, digital vibrance, and gamma ever-so-slightly to make my display look awesome again.

Thanks to ChrisF and BBlake for pointing me in the right direction."

"This sounds like the energy setting on a laptop behaves. Almost all Laptops have the option to dim down their LCD, but this is never found on stationary PCs to my knowledge. You may still want to check whether any such thing is configured in your energy saving settings, maybe by accident.

Next thing to do would be to try the display on a different machine. If it's dim there as well, chances are the monitor's lighting is broken."