I'm never sure why it happens but sometime Windows will freak out and you'll end up with a Window that is *completely* off the screen. When this happens it is impossible to get the Window back on your screen with your mouse so that you can move it around. But here's a keyboard trick you can use to get the windows back on your screen.

In the task bar right-click on the window that is off-screen and select "Move" from the menu. You can then use the arrow keys on your keyboard to move the window back onto your screen. When you are done moving it onto your screen hit the "Enter" key on your keyboard.

This is a great trick for rescuing windows that end up off the screen.