Once the Terminal window opens, enter the following command at the prompt:
gconftool-2 – -shutdown
(Note: There should be no spaces between the two dashes before shutdown.)
EDIT – Reader nickrud has suggested a better method instead of shutting down gconfd. Instead use the following command (thanks nickrud!)
gconftool – -recursive-unset /apps/panel
(Remember: There should be no spaces between the two dashes before shutdown.)
Then enter the next command:
rm -rf ~/.gconf/apps/panel
And enter one more command:
pkill gnome-panel
That's it!
Both top and bottom panels will appear (if missing) with their default settings. Now you can customize them to your preference and get on with using Ubuntu.