When you use the windows key + arrow keys (Move / Split windows) or switch monitors it can sometimes happen that certain program windows disappear completely. When this happens no shortcut is working any longer to get it back.
In older windows versions, the solution was right clicking on the program name in the task bar and choose move or maximize. But this option has been removed in Windows 10.
The solution is to open the task manager (Right click on the task bar -> Task Manager) or with
Ctrl + Alt + Del. Then right click on the missing application and choose maximize. This should bring the app back into the foreground.
Don’t use the shrink button again, just resize it ones manually, so the window remembers it’s new shrinkend position and not the old wrong one.