I've actually had that happen to me before too. 
On the USPS website it said that my package had
been delivered and signed for by someone else, in
a different state even.  Of course was not happy
at all and contacted the post office.  Apparently,
the website was wrongly updated and my cel finally
arrived in the mail soon after.

I would definately contact the post office about
this.  Hopefully your situation is similar to mine.