Posts are not normally deleted. However, we do move them if they are posted in the wrong section or remove them if they are inappropriate. You might want to check to see if it was moved to a more appropriate section. If you have one in particular that you are interested in maybe I can track it down for you and see what happened with this particular one.
Also like mentioned above, you may want to check your "Default Thread Age Cut Off" setting under User CP - Edit Options