Dont think it was a hot key. In fact how to Geek, tell how to create a hot key. I think it was a malfunction Improper shutdown, an install or a power surge are the most likely causes
Sometimes a game may change the desktop resolution to run, if the game exits in an incorrect manner it may leave your desktop in the same resolution that the game was running in.
What where you doing when the resolution changed?
What video card are you using?