Did you sleep well ? is the correct thing to say. (Simple past) Have you slept well ? is another way of using slept. (Present perfect) though they might be used in different contexts. Read more about these tenses to have a better understanding. The difference take s little bit of effort to understand. http://www.ego4u.com/en/cram-up/grammar/simpas-preper

Did you sleep well! Or did you sleep well last night