Sorry, I don't have an answer because I don't use Master Documents. However,
you may need the reason I don't more than you need an answer. "Master
Document" is a term of art in Word referring to a "feature" that not only
doesn't work but also destroys documents. The consensus (with the limited
exception of Steve Hudson) among those offering advice on these newsgroups
is that using the Master Document feature is a good way to destroy your
document. It can destroy parts of your document that you are not even
working on! I think John McGhie said it succinctly when he said that there
are two kinds of Master Documents: Those that are corrupt and those that
will be corrupt soon. See
http://www.addbalance.com/word/masterdocuments.htm for information on the
Master Document feature and workarounds. See
http://www.mvps.org/word/FAQs/General/WhyMasterDocsCorrupt.htm for more
information on what goes wrong, and
http://www.mvps.org/word/FAQs/General/RecoverMasterDocs.htm for ideas on how
to salvage what you can. See
http://www.techwr-l.com/techwhirl/magazine/technical/masterdocs.doc for
Steve Hudson's instructions if you are willing to follow them very
carefully.

You may be lucky if all you've lost is formats.