Custom world destroyed by a terrain formation

When I join to my city world  (which I terraform and build in it), I saw a giant mountain that destroyed my custom beach with all the buildings I put (a pier,a resort and all my beach) and I dont know what was the problem. I enter on my world before everything was fine but now its not. This took me at least 4 months and I want to have it back, please. My Minecraft my world name is Beach city (DEFINITIVO)

Help me to solutionate this problem. Here i put some photos of the incident.