You can find the sync.lrdata file here:
On Mac it's at /Users//Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata
On Windows PC it would be at C:\Users\\AppData\Local\Adobe\Lightroom\Caches\Sync Data\Sync.lrdata
Once done Restart Lightroom should try to reconcile your local synced data and the cloud synced data.
Let us know if that helps