Huge section of my castle complex was REPLACED by a rectangular cross-section of a mountain!

I was mining in my large underground mining complex, and then was riding my minecart up a long powered track to the surface when the track suddenly ended and my cart hit a huge flat wall that had not been there before. After recovering from shock, I looked around and realized that a rectangular cross-section of a completely different area had completely replaced the same sized section of my castle complex. There was tons of rock and dirt, grass and trees where just before had been a large portion of my castle. I lost many months of work and more treasure and resources than I can possible count because almost all my storage chests were in the lost section. I am so frustrated now that I am giving up this game forever after putting in thousands of hours of work on my domain.