It's really simple:

• the discussion owner, i.e. the user who has uploaded the discussed content or a group moderator, has deleted the comments. In this case, it can't be helped, as it is the wish of the content's author
• Also, the comments may have been deleted automatically due to a number of reports on them or the user who left them. We are sorry, but these comments cannot be restored since OK thoroughly keeps order on the website and doesn't allow it to be broken
• Or the website's Administration has blocked bad comments due to violation of the rules. And again, they cannot be restored. Rules are rules

In any case, the missing comments cannot be restored. The only exception is if the author of the comment has been temporarily blocked (and all his/her information has been ""hidden"" from the website). If the user restores his/her profile, it will resume its natural course and the comments will appear again.