If you want to fix the specific problem, you can install gconf-editor (for
instance from "Add/Remove Applications"), launch it (for instance writing
gconf-editor in the prompt you obtain by pressing Alt+F2) and mess up with
the values of the keys (I would look at /apps/panel/toplevels/*).
If you do not care about restarting from scratch the configuration of your
panels, you can execute, in a terminal:
$ rm -rf ~/.gconf/apps/panel
After a disconnection and a reconnection, you should have the default panels.
I have never tried that though!
