How to restore the default Lubuntu panel in LXDE based Lubuntu releases?
Found this at ubuntuforums.org, hope it helps:
cp /usr/share/lxpanel/profile/Lubuntu/panels/panel ~/.config/lxpanel/Lubuntu/panels
lxpanelctl restart
source
The other answers did not work for me (although to be frank I didn't log out, just lxpanelctl restarted) with Lubuntu 15.04 . This did work for me:
$ rm -r ~/.config/lxpanel
$ lxpanelctl restart
it may be a bit overkill though.
In terminal, execute the following.
cp /usr/share/lxpanel/profile/Lubuntu/panels/panel ~/.config/lxpanel/Lubuntu/panels
then logout and login.