If your gnome panel is missing, here are some steps you may try to get it recovered. (The following commands are tested in CentOS 6 environment.)
yum erase gnome-panel yum install gnome-panel reboot
After reinstall gnome-panel and reboot, you can continue to try the following steps if the panel does not come back.
Rename panel folder and reboot
Login as the user whose panel is missing.
mv ~/.gconf/apps/panel ~/.gconf/apps/panel.bak sudo reboot
After reboot, the gnome panel should come back.