Posts get deleted for lots of reasons. The mod or admin will move or delete a thread for being in the wrong forum, or delete a post completely for flaming, for discussing recreational use, for soliciting or offering illegal drug transactions, etc. Or the person who posted might have deleted the post themselves.

On some boards the entire thread will disappear if the initial post is deleted, I'm not sure if it works that way here too. Are your replies still there?

Read the guidelines, which goes over this territory pretty thoroughly.