Same here 
Desktop Dell Core2 Duo.
edit: I fix it.
Try just restart the panel first.
at first, try this:
killall gnome-panel
gnome-panel &
And see if it works.
I've done this (this will clean all your visual preferences, themes icons, etc.)
Procedure to follow 1) First you need to Boot the system upto Login Window and now Press Ctrl+Alt+F1
2) Now You will get a Text based login screen use your username and password to login
3) run the following Command from your terminal
$ rm -rf .gnome .gnome2 .gconf .gconfd
Now Press Ctrl+Alt+F7 you get your normal GUI login window
and then
killall gnome-panel
gnome-panel &
Try just restart the panel first.
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