First make sure you’ve alt-tabbed to the window, or clicked on it once to bring it into focus. Then right-click on the taskbar and choose Move.
At this point, you should notice that your cursor changes to the “Move” cursor, but you still can’t move anything.
Just hit any one of the arrow keys (Left, Right, Down, Up), move your mouse, and the window should magically “pop” back onto the screen.