If you are certain that the post has been deleted and you didn't do it, then there are two possible reasons: the message has been removed by a moderator or it has expired. Our boards are not designed to store messages indefinitely: after a certain amount of time older threads are removed from the board automatically. This process is called "expiry".
The rate of expiry is adjusted dynamically according to the posting traffic for each board. Threads are removed sooner from a board that attracts a lot of posts than they would be from a board with less activity.
Please keep in mind that the boards system is not meant to be an archival medium, and all posts are subject to expire and disappear sooner or later. This is intended to allow us to manage the resources consumed by each board effectively, and also to promote the ever changing flow of discussion.
Posts can also be deleted by an administrator if they have generated complaints from other users (through the Report Abuse feature) and have been deemed inappropriate. Please note that when an administrator removes a post that was the head of a thread, every other post in that thread is removed at that same time.
In cases of cumulative, serial or blatant abuse of the system, administrators may exercise the option of removing entire post histories from the system. If this has happened to you, its a definite sign that you are doing something inappropriate that has attracted multiple complaints.