Reseting Gnome Settings
If your gnome-panel corrupt , Application Menu, Places Menu and System Menu and some other menu items will disappear.
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.