It sounds like you might have disconnected your external display before going out of Fullscreen mode. You can resize the window. Simply click on the window and drag it up and to the left until you can see the bottom right corner. Next, click in the bottom right corner and drag up and to the left (this will resize the window). Then click the window again and drag it down and to the right (to move it back onto the screen). The window should be smaller now.