Follow the advice about the self addressed stamped return envelope in this thread first.  If you don't get it back.  Send her a certified letter respectfully outlining the attempts you have made to get your property back.  Tell her that she is unlawfully retaining property that belongs to you and this will be the last attempt to amicably resolve this matter.  Give her a deadline to send it back or contact you in writing about a method of exchange that works for her.