If something reappears, you can see it again after you could not see it for some time. A child hides, then reappears at the door. The moon reappears from behind the clouds. A worry reappears after a quiet week. The important piece is the gap in the middle: it was gone from view, and now it is back. The word does not say where it was, or why it returned — only that it is in sight again. You can use it for people, animals, things, sickness, problems, and even a song or a style of clothes that comes back years later. It is an ordinary word, not high or fancy. Though actually, older school stories love it for missing people and ghosts; younger people still use it for those, but also for a brand or a problem that returns. Disappear is the going-away partner.