A user posted a question that received negative comments and downvotes, and he promptly deleted it. The question was poorly formulated, but could easily have been sharpened to a clear mathematical question that had a precise answer; and I believe that this answer would have satisfied the user, who is now simply discouraged from posting. The user has over $1000$ reputation points, and obviously could be contacted via a comment to one of his other (unrelated) posts; but this seems to be a misuse of the comment system. Is there an approved way to encourage a user to repost a deleted question?
As mentioned in the comments, you can always contact the user via chat. If the question was deleted due to a bad reception or formatting frustration on the part of the OP, then contacting them via chat could be very helpful. Alternatively, sometimes people ask questions and then spontaneously decide that they don't actually want to ask the question. Obviously we aren't telepathic and contacting people via chat is a good way to find out their motivation, but you should be prepared for the fact that some users have little interest in reposting their question. If you like the question and think it would be a valuable addition to the site or wish to know the answer yourself, I would recommend you posting the question should the OP not respond or decline to repost the question.