I was in my world, in the process of building a massive structure. I turned off Minecraft and returned half an hour later, but over half of my structure had disappeared. This had also happened to another one of my structures, and every many of my other structures had completely disappeared.

I feel like it's important to clarify that I had backed up my world a few days before. I don't know if there's any way to restore it back to what it was when I backed it up, but if I could get some help and support with this issue I will be extremely pleased.