A family of Microsoft word processing software products for creating web, email, and print documents.
I just started having this problem about an hour ago. I have tried all of the above suggested solutions. The only thing that seems to work is to set the display to "duplicate displays" instead of "extend displays". That pretty much defeats the purpose of having another monitor.
Incidentally, about a week ago I started having a problem with Excel when I moved the display between screens when I had more than one file open. The displayed workbook starts vibrating and freezes up. It seems like this is likely related.