Yes. I did something, but I have no idea what I did.
Several things are wrong with my gnome desktop.

1. With that over, when I select System->Administration->
from the gnome panel, the Package Manager (synaptic) menu item has vanished.

2. When I select System->Preferences->Main Menu from the gnome panel, nothing happens other than the menu parts clear from the screen.

3. When I open the context menu (right click) of Applications->Edit Menu from the gnome panel, nothing happens other than the menu parts clear from the screen.

I could go on. I suspect [aka, hope & pray] that resolution to these items will clear or point the way to repair of the rest of things.

I tried to remove and replace the panel widget without success.

I tried re-install of various gnome parts without success.

I tried searching through various home folder gnome artifacts without success.

(grin, blush) It might help if I knew what I was looking for in the first place.

~~~ 0;-/

have you tried running update-menus to rebuild your menus ?