I'm not sure what caused this as I've never encountered it before.I believe Ubuntu was probably doing some updates and maybe a package like xrandr got updated and the brightness got set very low or maybe the Nvidia driver. Iwas using my computer, left it and came back and thought the monitor was off it was so dark. It also could have been related to a bug or issue with the screensaver/sleep mode when the screen starts to dim before sleeping after X minutes and........