For folks with multiple monitors, some applications remember what monitor they were closed in, and will always try to reopen in that monitor. 

If you have multiple monitors and a laptop that docks and undocks from that monitor this can be a real problem in windows, as the OS is not smart enough to control for this.  So even though the OS knows that there is only one OS, the app still opens up in a "phantom" monitor, and you can not access it or drag is back to the laptops monitor.

There is a solution!

Select the app that is in the "lost" monitor, and press the [windows key] + [shift] + [left or right button]

This will force the program to move to the other (aka only) monitor and bring the app back into view!