If the thread was in the Open Gate section then most likely the number of posts has exceeded 1 page and it has been moved to Central Street. This is done to encourage membership.
Or, if it was off-topic then it may have been moved to a more appropriate part of the forum.
Or, it could have been merged into another thread.
Or, someone may have posted inappropriate content and the thread has been pulled by the administrators for moderation. It may be cleaned up and returned, or it may not be returned at all, depending upon the nature of the content and the thread.