Hi, I'm posting just to see if anyone else has seen this happen and if so maybe learn from my experiments? Bold for the gist of it.
I have (had) around 14K photos synced from Classic to CC - I run CC on my Android phone and MS Surface, Classic on my windows desktop.
I just opened LrCC on my phone to find almost all of my collections are empty and I now have just 19 (unrelated in any way) synced photos. This is also the case on my desktop Lightroom Classic, and on the Web, and when I turned on my Surface, it rapidly synced from an initial 14K or so photos, to the same 19 photos. I guess I had no option but to let them all sync and that's where I am now. The photos are safe on my PC with 2 backups so I wasn't worried about losing the actual photos. And I backup my catalog every session.
I have not been doing any bulk work on my collections and it seems almost impossible that I inadvertently selected all of my photos, except for this random 19, and then removed them from my collections. I can't think of any other way this could be a user error but happy to learn otherwise.
I am currently attempting to restore from a backup catalog but in the process of shutting down Lightroom Classic I've found that the "optimising catalog" is running for much longer than normal - maybe 5 minutes so far - and counting - and it's chewing up RAM - 22GB - I think it's crashed but I'll give it a while. And....Lightroom just crashed, monitors went blank, unplugging one of them restored display. Sent the crash report to Adobe.
So I'm really hoping restoring the backup will do it, I don't relish the task of re-populating all those collections!
Please post if you've seen this problem too, or can offer any advice. I'll update as I go.
OK, backup catalog now restored but sync wouldn't work until I deleted the sync from the cloud and the pc - it took a few goes but I'll keep this as brief as I can.
So now I'm resyncing 14K photos, and rebuilding the folder structure manually which is a complete pain in the xxxx. I'm now behind with work, so a frustrating waste of time, I hope this helps others, and if there was a shortcut I'd be grateful so I know for next time.
Don't forget those backups - I have no idea how long it would have taken me to re-catalog all my photos...and I don't fancy finding out!