If you’ve mangled, deleted, or otherwise borked your Ubuntu Gnome panels, here’s a way to revert them back to the default appearance and positioning.
You’ll want to open the ‘run’ dialog via ALT-F2 and enter each of the commands one at a time, followed by enter.
gconftool --recursive-unset /apps/panel rm -rf ~/.gconf/apps/panel pkill gnome-panel gksu reboot
That should do it. When the system comes back up, all should be back the way it was.