This happens from time to time. Gnome has just lost your window manager for some reason. (Check your quota) To fix it...
On the tool bar at the top of your window click...
In the box that opens up scroll down on the right to 'Desktop' and choose the option 'Window Manager'.
Choose Twm from the left hand window.
Click 'Save session later.'
Now choose Sawfish.
Click 'Save session now.'