I closed my file and opened another. Then I closed that one and opened my first file. The Sync button was greyed out. Then I closed FTM and it ran a sync. After I re-opened it (first it said that my file was in use by another process, then opened it) and sync was now available. I checked ATM and today's changes were online.
It appears to have recovered. I compacted and backed up my file right away!