Many Ubuntu users would customize their gnome desktop by changing the default panels with dock or AWN.This tutorial shows how to restore this default panels.
First,open terminal from Applications->Accessories->Terminal.type this command and all panels will disappear.
gconftool --recursive-unset /apps/panel
then,type this to delete current one:
rm -rf ~/.gconf/apps/panel
Finally,reload panel:
pkill gnome-panel