1.9.1 pre3 worlds self-deletion

I spent many, many hours playing on a 1.9.1 world, spending hours upon hours gathering resources etc. A large amount of mobs were spawned near me, so many that it began to lag out my powerful computer. Suddenly, the game crashed, and upon relogging, the world had ceased to exist. I checked my saves folder, and it was nowhere to be found. Very annoying bug, wasted a lot of my time.