The same just happened to me. And I did not delete anything recently. I entered Zotero in my Iphone and the collection was there but I made the mistake of allowing sync to occur and the collection disappeared in front of my eyes.
(Please start new threads for new issues — posting to years-old threads opened by others isn't helpful to anyone. I've moved this to a new thread.)
Syncing doesn't delete things — it just syncs changes you've made elsewhere.
In this case, you deleted the collection in the desktop app shortly before posting here — apparently by mistake — and synced that to the online library, where it then synced to your iPhone.
Since you just did this, you should be able to restore the deleted collection from one of the automatic backups. Note the specific instructions for restoring collections.
Also note that the items are all still in your library root, even though you deleted the collection.
Zotero 7, currently in beta, will make it possible to restore a deleted collection from the trash.
@wboeger: Actually, sorry, it looks like you deleted a collection from your Mac (using Zotero 6) shortly after posting here.
You deleted another collection a couple days ago, on the 24th. That was also from a Mac, but it was from the Zotero 7 beta. Since the beta now sends collections to the trash first, and it doesn't automatically purge collections from the trash, you would've had to have purged that collection from the trash explicitly (e.g., by purging it directly or by emptying the trash).
In any case, if that's the collection you're referring to, it's possible that one was deleted before your automatic backups, but you can check the timestamps of those files to see. Again, the items are all still in the library root.
Upgrade Storage
Syncing doesn't delete things — it just syncs changes you've made elsewhere.
In this case, you deleted the collection in the desktop app shortly before posting here — apparently by mistake — and synced that to the online library, where it then synced to your iPhone.
Since you just did this, you should be able to restore the deleted collection from one of the automatic backups. Note the specific instructions for restoring collections.
Also note that the items are all still in your library root, even though you deleted the collection.
Zotero 7, currently in beta, will make it possible to restore a deleted collection from the trash.
You deleted another collection a couple days ago, on the 24th. That was also from a Mac, but it was from the Zotero 7 beta. Since the beta now sends collections to the trash first, and it doesn't automatically purge collections from the trash, you would've had to have purged that collection from the trash explicitly (e.g., by purging it directly or by emptying the trash).
In any case, if that's the collection you're referring to, it's possible that one was deleted before your automatic backups, but you can check the timestamps of those files to see. Again, the items are all still in the library root.