[ubuntu-uk] How to fix Gnome problem

[Mac]

I've been running Dapper with Gnome on a Dell Inspiron 8200 laptop with 
no problems.  Recently, when I click the Log Out button I get a pop-up 
saying 'Your sessions has been saved'.  I click OK on this pop-up and it 
closes, but the normal log out dialogue does not appear.  When a click 
the log out button again, I just get returned to a fresh log on screen.

The only way I can quit is via the System menu, or the options menu on 
the log on screen.

I'd be glad of suggestions about how to repair.

Mac