Someone deleted the items, or someone left themselves logged in on a shared computer and someone else tried to start using Zotero from scratch (but that would generally result in all items being deleted).
If a group admin emails support@zotero.org with the group name, we can tell them more.
I'll have the admin email. No one deleted the items and computers were secure. The only thing I can think of is that it has to do with updating to the latest version of Zotero.
Upgrade Storage
If a group admin emails support@zotero.org with the group name, we can tell them more.