iPhone stuck indexing in Ford Sync

I bought a 2013 Ford Focus in September. My iPhone 4 (current iOS) has worked perfectly with Ford Sync connected via USB, playing my music without a hitch. Then, about a week ago, when I selected USB with my iPhone conencted as I usually do, it got stuck in a perpetual "Indexing" mode. I have tried Master Reset several times to no avail. There does not seem to be any updates available for my SYNC version. Any thoughts? Anyone?