Comment 10 for bug 1015297

And amazingly I do have a workaround! Well, for AMD/ATI users at least.

If you have the proprietary AMD drivers installed, open up the Catalyst Control Center and switch over to the "Color" screen. Tweak the "Gamma" setting (make sure "All(locked)" is selected under "Select Color Channel:") and the screen should pop back into full brightness. Hit the reset button for that slider (off to the right-end of it) and it should stick.

I'm not sure if this is applicable to Nvidia users, or users without proprietary drivers. Nvidia users should definitely try this if they have their proprietary drivers and have access to their equivalent of AMD's CCC (don't remember what it's called for Nvidia cards). Non-proprietary driver users should look into installing Redshift (run "sudo apt-get install gtk-redshift") to graphically tweak the gamma settings.