Have you tried moving the cursor into the upper right-hand corner?
If you press F11 in Windows you will enlarge the application window by removing the toolbar. If you press F11 again, the application window should return to its previous size. While in F11 mode you can move the mouse cursor into the upper right-hand corner. This will reveal a small drop-down toolbar that will allow you to restore the application window.
If this still doesn't work, try restarting the computer. If you haven't touched any buttons, it might simply be a minor hiccup.