Chunk reverted

I was mining away from my home (base) chunks and when I resurfaced and came back to those chunks, I found half of my base missing.  So once I turned on chunk borders on, I realized the missing area in my house was a single chunk reverted to the 'original' state of the world.  Meaning from Y: 0 - 256 in a single chunk was replaced with the same grass, stone and cave stuff as when I first loaded up the world.