If a window gets out of the visible area on your screen(s) it seems to be hard to get it back into your sight.

But it is not that hard:

• just select the missing window, press ALT+Space to get the contextmenu on the missing titlebar.
• select Moving (or press M)

as you might not know where the window is at all, moving it by the arrow keys might get thing worse. So:

• press an arrow key once, then move your mouse without clicking. There the window is again! Click, when you want to drop the window at the current position.

Done!