If you accidentally removed the panel in Ubuntu (I believe it should work with any distro with GNOME desktop), follow these steps to restore the default GNOME panel:
Open terminal (if there is no place to click, press Alt-F2 and run gnome-terminal).
gconftool-2 –shutdown (OR gconftool –recursive-unset /apps/panel)
rm -r ~/.gconf/apps/panel
You should get your default panel back.
(Ref: www dot watchingthenet dot com)