my world automatically deleted itself

Worlds are not backwards compatible with older versions of the game. Equally, as indicated with the notification, using snapshots and pre-releases may cause corruption or unexpected side-effects, so it is not recommended to use your normal worlds in these.

so what should I do?

Do not open worlds that you care for in prereleases or snapshots, and do not open worlds in an older version than they have been last opened in. In both cases, the world might get corrupted. Unless you have created a backup, there's no way to restore the world.