I seem to have had this same problem. In preparing a historiography course, I had a huge folder with subfolders. They all seem to be in my library (at least a couple I have checked). But the folders that they were in seem to have disappeared. I have looked to see if it got moved by accident to another folder as a subfolder. But no. I might have clicked delete on an item and somehow it slipped to the whole folder. But there seems to be no failsafe on this (which, in laptop world must happen a lot).

Judging from the thread there is no Undo. Are these the correct steps for the future?
1) Create these all again by looking through my entire library (uggh).
2) Read more closely and figure out how to save my Zotero libraries periodically.

Whoops. Sorry (in my mind the folder with subfolders WAS a collection, but never mind). But did the sqlite.bak to sqlite thing and they're back! Well, no. They are back for a few seconds, in fact enough time for me to export the folder. But then they disappear again. (I suppose the cloud has already updated with deleted folders.) I'm going to keep doing this and exporting the subfolders. Hoping then I can import again. So many thanks. I think.

Hello - I'm in the same boat. I mistakenly deleted a collection (not the items) and want to restore it.

I have .bak files that have the deleted collection, but they won't have a number of changes that I made to the database today. Is there a way to use those files to restore the deleted collection without losing the changes I made today?

Also, I turned off auto-synch and can see the deleted collection online. Is there a way to integrate the online collection into the database on my laptop, without reverting to the last online synch (of the full database) and losing the changes I made today?

Thanks!

PS - Looks like that undo for accidental collection deletion is a needed feature!

Thank you, but I'm not sure I fully understand.

When you point to the last automatic backup steps, are you saying I should do this: "close Zotero, go to your Zotero data directory, and rename it (e.g., to “Zotero-old”). Then, re-open Zotero and sync from the Zotero servers to the new empty local library."?

Or are you saying I should follow the alternative steps: rename zotero.sqlite to zotero.sqlite.old and then rename a .bak file so Zotero loads from that file upon restart?

I think you mean the latter, since you quoted my statement about having .bak files. But why load a .bak file and sync it to the online backup if the online backup already shows the deleted collection? If the sync works as I understand you to be suggesting, couldn't I just sync my current database (which ahs the new changes but not the deleted collection) to the online backup (which has the deleted collection but not the new changes)?

(Sorry, that other part shouldn't actually have been there, and I've removed it.)

Check under unfiled items - my citations went there after I inadvertently deleted a collection.