[AnotherMuggle]

To move a window whose title bar is not visible, click the window or its corresponding taskbar button and press Alt-Spacebar.

Even if the window is entirely off-screen, not visible at all, the system menu will appear as close to it as possible, so you can use the menu's location as a pointer to the "lost" window.

Press M (for Move), use the arrow keys to bring the window back into view, and press Enter when you have it back under control.