I just upgraded to Monterey and lost use of my second screen. Tried everything said here as well as in Apple support. On a whim, while in the display settings I dragged the uncooperative screen to the left side and it works! At least I can move my mouse back and forth and the dock moved left. Windows open using either screen now. The only thing is that I didn't have the same picture so went back to select it. I'm really hoping it will continue to work this way.