Sometime things break. It can be Compiz, the Gnome panels and so on. And if you cannot access their options, you cannot remove the faulty settings so one way to solve this would be to completely reset their settings.
But I've actually wrote this post for something trivial: because I receive A LOT of emails asking how to restore the sound icon (part of Indicator Applet) or some other Gnome Panel applets / indicators to their original state. Well, here it is: