how to reset gnome panel?

Config reset

This worked for me in GNOME Classic on 12.04 to reset the gnome-panel configuration:

dconf reset -f /org/gnome/gnome-panel/
killall gnome-panel

Run the commands in a terminal or with Alt-F2.

Popup menu

Try Win-Alt-rightclick to get the popup menu.

Open a terminal and paste:

gconftool-2 --recursive-unset /apps/panel

and

killall gnome-panel

Or you can manually delete the folder ~/.gconf/apps/panel/

mkdir ~/gnomepanelbackup ; mv ~/.gconf/apps/panel ~/gnomepanelbackup

this will create backup of current settings in home directory. then,

killall gnome-panel

Reference: 1.