If it is so dark that you cant see the screen then restart your pc in "safe mode" then change your settings back. Safe mode will not be using your video card current settings.
If you know the Fwhatever key that your pc needs you to click during boot up to get to safe mode great, if not, start your pc, let it boot all the way to windows, then turn off pc with the power button, then restart, when it restarts it should tell you that you had an incorrect shutdown and offer you to start in safe mode to correct the problem that caused it. Hope this helps.