When something disappears, it's gone. To disappear is to vanish, evaporate, or just fade away.
The word disappear is made up of dis, meaning "do the opposite of" and appear. So to disappear is to do the opposite of appear. Magicians are experts at making things disappear: one minute the rabbit is here, and the next, it's gone! When you lose something, like one of a pair of socks, you could say it disappeared. Snow disappears as it melts; boiling water disappears as it evaporates. Anything that goes away without leaving a trace has disappeared.