Menus missing stuff, how to fix?

[Scott I. Remick]

Shortly before I upgraded Gnome, my Applications menu got corrupted and
stopped working. Clicking on it would just produce a tiny box beneath it
with no contents. After upgrading Gnome at first it seemed like the menu was
working again but now I realize (yeah I don't use the menu often) that a lot
of standard things are missing. For example, my Accessories menu only
contains 8 items and is missing stuff like the Themes option.

Is there an easy way to straighten this out? My first choice would be a way
to merge the "standard" stuff back in with what's there now. A
less-than-ideal but workable 2nd choice would be a way to reset it entirely
to the default and I add my custom stuff back in on my own.

Thanks