I had a discussion proposing a game. (to post audio links on the discussion) And my discussion is gone now, I can't see it. Maybe a moderator has removed it for some reasons I don't know, but I had no notifications; It's the mess in the discussion, I wish I have one place with all my discussions.
Generally, when a discussion gets removed the reason is that it is against the guidelines. It is not possible to notify every user why a discussion is removed (believe me, as a moderator who deals with tons of spam daily, it is simply not feasible. EDIT - I am not saying yours was spam necessarily.).
Of course sometimes errors occur. Have you read the guidelines in detail and made sure that it does not violate any of them? http://www.duolingo.com/guidelines