I don't have a solid answer for you. However, after making sure I had backup(s) just in case things got worse, I would try the following,
1. Sort log by callsign. Presumably a blank callsign will show up at the start (or end) of the display
2. Export the entire log to an ADIF file, make a new logbook and try importing the ADIF to that logbook.
3. As a variation of step 2 above, you could also try importing old backups into a new logbook to try and get back to a stable state.