There are many reasons why your post(s) could have disappeared:
Not An Update
If you posted a non-update in the BB Updates forum, your post and it's replies are removed. If you are not sure where to post, then read the BB forum Guide for more information.
Due to the traffic and activity within the BB Discussion forum, posts are expired from this forum on a regular basis. Only the last 2000 topics are guaranteed to be kept at the time expiring takes place. Unfortunately this leaves us with only 1-3 days of activity in the BB Discussion forum normally. During events that draw high traffic to the forum, this amount decreases even more. Other forums may be expired but hold weeks of history still (depending on post counts).
This will effect posts you have marked as favorites, as they will be removed as well.
Moved or Moderated
Your post or the post(s) you replied to were moderated, removed or simply moved into the appropriate forum. In most cases if you started the topic you will receive a Private Message notice if your post has been moved or removed. A pointer may or may not be left in place of moved topics.
Edited (02/26/13 03:23 PM)