Do you have more than one catalog? Try from the menu bar to look for named catalogs:
File >> Open Recent
I tried that. There's no other one...
Do you have a backup you can return to e.g. from before the date of the last sync?
If so, unzip the folder and extract the files, then double click on the catalog (.lrcat file) to launch.
It could be that if you syncd folders - you re-imported older images and lost the metadata.
No Backup. I don't know why but it with the update on lr 6 it stopped making backups itself..