You can email team@discourse.org in this case (though the community is also good to help), but I think they usually don’t work on weekends.
You can’t delete a category if you don’t delete the topics in it beforehand.
Either you deleted an empty category, in which case you can just re-create it, or you actually deleted the content before this, in which case you need help from the Discourse team and see if they can restore the lost content.
Edit: I’m not sure you can restore a deleted category with the same ID, tho 
Yeah, probably not: Recover deleted categories - #4 by pfaffman