If you just did this, you should be able to restore a deleted collection from your last automatic backup.
Note that, unless you really went out of your way, the items themselves are likely still in the library root or the trash, but the steps above will explain how to restore collections specifically.
A future version of Zotero will make it possible to restore a deleted collection from the trash.
Thank you. Fortunately, I have the habit of tagging with keyword same as name of collection. I am aware that the assets are still intact. To be able to restore from trash will be great, this is IMHO a critical missing feature and I post after finding nothing about 'undo' for collection in the forum.
Upgrade Storage
Note that, unless you really went out of your way, the items themselves are likely still in the library root or the trash, but the steps above will explain how to restore collections specifically.
A future version of Zotero will make it possible to restore a deleted collection from the trash.