Every once in a while for a variety of reasons, it’s possible for a window to get positioned such that its title bar – the bar across the top of the window with the program name as well as the Minimize, Maximize, and Restore buttons – is off the screen, becoming both impossible to see and impossible to click on using the mouse.
Almost as common are windows that are completely off screen – perhaps due to unplugging a second monitor before moving all windows back to the remaining screen.
If you’re used to using the controls on a window’s title bar to move, manipulate, or otherwise deal with the application, you’re kind of out of luck.
At least if you’re using a mouse.
It might be time to break out the keyboard interface.