Assuming you're referring to a collection, if you just did this, you should be able to restore a deleted collection from one of the automatic backups. Note the specific instructions for restoring collections.
(Also note that the items are most likely still in your library root, even though you deleted the collection.)
A future version of Zotero will make it possible to restore a deleted collection from the trash.
Upgrade Storage
(Also note that the items are most likely still in your library root, even though you deleted the collection.)
A future version of Zotero will make it possible to restore a deleted collection from the trash.