Just a tip I had to share. Ok so you are taking your computer somewhere else and you unplug one of your dual monitors. When you arrive you set up your computer with only one monitor and start using programs only to find that a SUB Window is open ON the monitor that is no longer attached. NOT only that but a sub window doesn't have a task bar icon so you can't select it.
This should fix it. Do your action to make the window active. In my case pressing the "render settings" button in Maya pulled up a sub window.
Now hold shift/windows key and then the numeric arrow left and right keys on your keyboard. It will instantly jump to your screen. Worked for me with a Maya sub window that didn't have a task bar icon.
Hope it helps you out!