If you're using Zotero 7, you can just restore the collections from the trash.
If you're still on Zotero 6, and you just did this, you may be able to restore from one of the automatic backups. Note the specific instructions for restoring collections — don't use Replace Online Library.
Either way, unless you really went out of your way, the items would all be in your library root, even if you deleted a collection.
Upgrade Storage
If you're still on Zotero 6, and you just did this, you may be able to restore from one of the automatic backups. Note the specific instructions for restoring collections — don't use Replace Online Library.
Either way, unless you really went out of your way, the items would all be in your library root, even if you deleted a collection.