This falls into the category of frequent and annoying problems! I end up Googling this every time it happens, and here is how you fix it. This works even if you use a multi-monitor task bar like DisplayFusion offers.
- Use ALT+TAB to switch to the off-screen application.
- Press ALT-SPACE to bring up the system menu (you won't see it because it is off screen)
- Press R to select the "Restore" menu choice to ensure the windows isn't is maximized (you can not move it if it is maximized)
- Press ALT-SPACE again, then M to select the "Move" menu choice.
- Press one of the arrow keys to initiate the movement.
- Now just use the mouse to place the window where you want.