… Ok, I’m getting doubtful here, are you sure?
I think either “How was your sleep?” or “How did you sleep?” would be right, even “How were you sleeping?”, but not what’s there; if you have the word “was”, I imagine “sleep” as a noun and “your” as a possessive determiner, while with “did” I interpret “sleep” as a verb with “you” as its subject; I can’t make sense of “How was you sleep?” though.