Try launching FileMaker Pro 14, enter Preferences and hit "Reset" where it says "Reset dialog sizes and positions:". Let me know if this resolves or if you have further information.
If you are running a windows machine and run into this issue with a couple of the different windows like debugger and script workspace in 14 there is another way you can try to recover it as well.
If you click on tools and then launch script debugger and it doesn't show up hold down your windows key and press the left arrow a couple of times while still holding down the windows key. If your screen was on a second or third monitor that no longer exists it will move the program back to the main screen.
I connect and disconnect monitors from my laptop a lot so this is a fairly common problem for me.
The rest works great too, just wanted to point out another alternative.