Chunks Reset and Scrambled in minecraft 1.9

Left my single player survival world last night, went back on it this morning to find myself suffocating in a wall, then re-spawned close by to see that the chunk had not only reset, but completely changed, being around 20 blocks lower than the rest of the terrain, not only this but around 30% of the chunks within a 10 chunk radius of this point had also scrambled, not sure what has happened.