That's why I keep a "center window" hotkey in my main AHK script...for those windows that get stuck off-screen. Or, you can use this procedure to bring off-screen windows back.
1) Focus the window by clicking its taskbar button.
2) Press alt-space, m, down-arrow.
3) Now, without clicking your mouse at all, move the mouse around. Your window should be stuck to the pointer and easy to retrieve.