This happens quite a lot to me too. All you need to do is Crtl-Alt-Delete and open up the Task Manager and then go to process and find freelancer.exe and end process. Don't do this too often though, only do it when you need too.
It's a bug that happens every once in awhile. No idea how to fix it, and it doesn't really happen often enough to be a problem anyway.. Only way to get out of it is to close FL and restart it.