Worlds keep getting reset.

I have been working hard on survival worlds. I keep trying to load it and its says,"problem loading world". When I try a second time it just resets the world! Like as if I had created a world with that seed. Weeks of work going away in seconds! This happens every time I make a world. The issue is more likely to occur when I load a resource in the world. So if you fix this I would really appreciate this.