Lets try a few things!

-Use ALTTAB to switch to the off-screen application.
-Press ALTSPACE 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 ALTSPACE 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.
-If you are using a non-english version of Windows, the "R" and "M" menu choices probably will be different.

--For Windows 7 users: Win + Shift + Left or Right will move the selected window to the monitor in that direction.

If that doesn't work please report back!