Close OpenOffice.org completely (including the quickstarter, if it is running).
On Windows, open up the folder c:\Program Files\OpenOffice.org 3\Basis\share\config\soffice.cfg\modules\swriter.
On Linux, look in /opt/openoffice.org/basis3.0/share/config/soffice.cfg/modules/swriter/menubar/.
I have not access to a Mac so cannot say where the file is kept in its profile.
Tip: To modify Calc instead of Writer, substitute scalc for swriter and similarly for the other OpenOffice components.
Tip: The path varies for OpenOffice.org version 2 and on many Linux systems, so if you can't find it, do file search for swriter.
Is there a file in the menubar directory called menubar.xml? If not, that may be the problem. A new User Profile should recreate the standard menubar, but if it doesn't I'm sure some Mac user can supply you with a copy of their menubar to insert; possibly a menubar.xml from any version of OpenOffice Writer will do, but best to use a Mac version just in case.
If there is and you rename menubar.xml to menubar.xml.bak your menubar will be missing next time you start OpenOffice.
This information is based on http://www.oooninja.com/2009/03/april-fools-jokes-pranks.html