How can I force macOS to show the window of a program that I know is open but won't appear anywhere?
Have you tried using Mission Control?

If the window is indeed open, it should show up there.

How to use Mission Control:


Swipe up with three or four fingers on your Multi-Touch trackpad or Magic Trackpad.
Double-tap the surface of your Magic Mouse with two fingers.
Click Mission Control in the Dock or Launchpad.
Press the Mission Control key on your Apple keyboard, or press Control+↑.
In OS X El Capitan, drag a window to the top of the screen.


Adapted from Apple's Support Page.