Ubuntu gnome panels are nice enough to arrange shortcuts, menus, items on it. Even new panel can be added on gnome desktop‘s left, right or bottom side. But some times a panel can be customized or assembled roughly. Then you would like to reset all as its default sate. For that, open Terminal and run these following commands:
gconftool-2 --recursive-unset /apps/panel pkill gnome-panel
It should be restored on its default settings!!!