To move a windoow around using the keyboard.
Before clicking on anything else (so that that widow retains the focus) hit Alt and Space. This will bring up the system menu. Hit M to move and then try each of the cursor keys in turn to see if your lost window reappears on one of the screen edges. Once you have got enough of it on screen to grab with the mouse, hit Return to turn off the keyboard controlled movement.
If the window is normally maximized, the move option will be greyed out and you will need to hit N to minimize it first, then move it.