I had the hardest time with this issue, 2 crucial parent and child windows were lost offscreen.  I needed to get them back, then I found this simple fix here (thanks Kevin, whoever you are).  I'll repost the content in case that link stops working:

1. Hold down Alt + Spacebar
2. Press the M key
3. Use the arrow keys to relocate the window
4. Press the Enter key when you have the Window in the desired location.