[Help] Lost World

Hello me and my dad have a world that we have been playing and building stuff on since 2013 on xbox 360. We are on xbox one now. He was just playing on the world an hour ago and when he got back the world is no longer there. We have done so much on that world just for it to disappear like it never exist. He is also going through a tough time right now with life so this isn't making it better. He called me crying telling me that it was gone and he can't find it anywhere. He lost his internet a couple months ago so he has been playing it offline since then without me until now it just vanished.

Can someone please tell me how we can get it back and why it disappeared?