Single-player world loading incorrectly. Put together like a bad jigsaw puzzle.

I logged into my single player world this morning and the world was all screwed up. It's as if the elements of the world merged together, with elements opening in the wrong places. The outer wall of my cave is gone and has been replaced by a wall of water. My excavating areas have disappeared too. The world was fine last night when I finished a gaming session, then it was like this when I logged in this morning.