- Make sure, the invisible (off-screen) window has the focus: Hold down ALT + Tab to select the (invisible) window you want to move
- Hold down Alt + Spacebar
- Press the M key
- Use the arrow keys to relocate the window until it becomes visible again
- Press the Enter key when you have the Window in the desired location.
via Move a window when its title bar is off the screen (Tips, Tricks, Tweaks, and Setting) – TACKtech Corp..