Reset Gnome.
First of all, backup your settings.
mkdir ~/backup_gnome
mkdir ~/backup_gnome/.gnome/
cp -R ~/.gnome/* ~/backup_gnome/.gnome/
Do this with these folders: .gnome, .gnome2, .gconf, .gconfd, .metacity
Another way is to backup ALL of your settings:
mkdir ~/backup_settings
cp -R ~/.* ~/backup_settings/
After making backups, you can delete all gnome settings:
rm -rf .gnome .gnome2 .gconf .gconfd .metacity
WARNING: If you remove all these folders, you essentially remove all the settings.
After resetting gnome, Logout and Login again (or restart your computer). You panels should be restored, now you can edit them again.
I hope this helped you, but try all the answers before mine, because resetting gnome is like reinstalling gnome.
Daniel
This apparently results from a 10.10 repository error where nvidia-current update was botched and as a work-around I installed driver from nvidia. This resulted in a botched 10.10>11.04, then a botched 11.04>11.10, then a botched 11.10>12.04 release upgrade. It kind of incredible that the resulting 12.04 would boot at all.
Remarkably, the same thing on a 32bit system worked perfectly.
The solution apparently will be to fix the 11.04 so the following upgrades will work, if that is possible.
Fortunately I have backed up each step.
UPDATE:
Mismatch seen after upgrading without nvidia-propietary:
UPDATE This now seems to be associated with 12.04's inability to support a second monitor (at least in amd64 with nvidia graphic adapters). These occur when adding/deleting 2nd monitor (screen1) using nvidia settings and logging on/off. Bug reports have been filed, Bug #1002641 and Bug #1001712 and possibily related to Bug #1002602 and Bug #729979
This also occurs with Unity.
UPDATE Bug #1002641 is getting a little attention but nobody is assigned to work on it. After today's update, I now have the Unity Dash Totem on my Gnome fallback/classic screen obscuring the Gnome top panel and pretty much making 12.04 useless. Thank goodness I have a working 10.04 also running so I can get some work done.
Best Answer
Well , I had script for resetting the gnome panel . I am not sure if this script will work on notebook version , i used it in desktop version ,and its very helpful .
save this script to PanelRestore.sh to the desktop for example and
chmod +x PanelRestore.sh
and then run it by terminal./PanelRestore.sh
, a new window will pop up , just click restore default setting and you would be good to go