World deletes itself.

Yesterday my 3-month-old world disappeared. I did everything I could think of and it didn't work. Later the same day I created a brand new world and I was already proud of it, only for my monitor freeze. I turned my computer off and back on, and loaded up Minecraft, only for my world to be gone. Mojang please fix this, this is possibly the worst bug in Minecraft history. imagine if my world was three years old. Mojang, this bug should be your top priority.