Dissapearing Survival World

I made a survival world on singleplayer, there was nothing wrong with it, but once my world just dissapeared from my singleplayer list.
I was soo suprised that I made a new survival world with exact the same name and everthing and I spawned in my world. All the stuff that I got in my inventory was gone.
I don't understand why this worked but I'm happy.
Now my world dissapeared again and I did the same thing, I made a new survival world with exact the same name etc. and it worked* again*. All my stuff I got was also gone.
I don't know whats happening with my survival world but I'm realy angry what happend. I'm soo scared that it will happen AGAIN en I have too collect alll the diamond agaaain.
Do someone have the same problem?

h3. Someone please read this!