A discussion may be missing for several reasons:

• If the discussed content (post, photo, video, topic, etc.) has been deleted
• If you have hidden this discussion. In this case, it's very easy to restore it: just find the post, photo, topic or video you have discussed and click ""Comment"".
• If the discussed content has been blocked by the Administration for some reason.

It is impossible to know for sure where the discussion went. All possible reasons are listed above and you may quickly guess what has happened to the discussion.