Nancy, when a ferret dies of unexplained causes it ALWAYS pays to have a
necropsy done by the vet and pathologists.  This allows you to eliminate
disease and poisoning concerns which could affect other ferrets' wellbeing
and longevity, or to respond appropriately if such things are found.
 
Dan, the very deep sleep is worsened by insulinoma.  Might pay to consider
a blood test; ask vet for opion on testing now or later.  If she has been
less active recently or staring off into space at times, or any of the
other early symptoms you especially need to arrange this.  If you aren't
familiar with insulinoma you will find marvelous info at:
http://www.ferretcentral.org, http://miamiferret.org/fhc, and
http://www.afip.org/ferrets/index.html .
[Posted in FML issue 3074]