This happens often and it’s aggravating. You attempt to switch to an application say by clicking on it in the dock, or attempting to open it, only to realize it’s already open, but you can’t see it.
I think this may happen when you switch from target display mode.
Anyways one fix I’ve found, at least for some apps is to select the app icon in the dock and then click “window” from the menu bar and select “zoom.” That should bring the app back on screen.
If you find another way around or if this doesn’t work for you app then please leave a comment.




