This might happen when you have dual-monitor setup, press alt-tab to bring the focus on the window, the press alt-space to activate window control menu, choose "move", now there you go!

credit: http://blog.vogella.com/2013/06/02/ubuntu-how-to-move-an-application-which-is-displayed-offscreen/

Not exactly an odd issue, happens so often. Switch to console, run `setsid unity`, this "might" fix it, if not, reboot your desktop :D