It's not uncommon for Windows to lose display settings, especially with "plug and play monitors". What I'd do is boot to safe mode, and remove all instances of monitors in device manager. You probably now have a couple of defunct entries in there. I have that happen once in a while, Windows detects a "default monitor". If I go to safe mode, I have an entry for default monitor and Mag XJ500T. I usually remove them both/all (but in my case just removing the default monitor and restarting normally is sufficient)..

One example I know of that can cause things like that to happen. Games, when they mode switch sometimes.