Here is a great tip on how to recover it:
Boot the system upto Login Window. Press Ctrl + Alt+F1
Now You will get a Text based login screen
login with your username and password
now you will get a $ prompt.
Type the following Command
$ rm -rf .gnome .gnome2 .gconf .gconfd
Now Press Ctrl + Alt+F7
and login Gnome. Every thing appear in the menu bar.
Source: http://www.ubuntugeek.com/ubuntu-tip-howto-recover-gnome-panel.html
