Half my world reverted back to original seed

Everything I had built on one half of my minecraft world (everything positive z) suddenly reverted back to the way it was before I built, terraformed and generally did anything. The original seed over-wrote all of my progress on the entire positive z half of my world. I don't know exactly what triggered it, but I think it may have been me trying to export a copy onto a flash drive. I later found out that all my backups for the past two months were also nullified, so my backup (what I tried to export the world to) had lost two months' worth of progress.