My Worlds getting deleted

Hello, I have a pretty major issue that has happened to me more than a couple of times.

• I start a new game and after I get pretty far in the game, it would get deleted.
  The world is fine up until I have quite a lot of items and quite a lot of buildings that i have spent my time on.
• I have had this occur to me twice.
• I have other worlds that I have had for a while now but dont play as much and they are perfectly fine. I don't know what to do.
• Due to this issue i never have any worlds that i can play for a while.