I've just done something to my computer and don't know how to undo it. Basically I've got a full screen now, no address bar on top and no bar along the bottom. If I move my mouse to the top of the screen the address bar re-appears, but when I move the mouse away it goes again.
And the 'start' in the bottom left hand corner has gone and no matter what I click it won't come back. I've got to close all open tabs to see it.

Help me before I restart it.