When i try to open my mine craft (survival) world it will not open. When i attempt to open the world in minecraft it sends me back to the main menu. I had installed 13w42b the previous day, not liked it and removed it. I returned to 1.6.4 and everything was fine. I opened the world several times after un-installing the snapshot, it seemed to work fine all these times. I tried copying my world back into my saves but this didn't help. (I was experiencing quite a bit of lag, which is unusual for me, the last time I was able to play on my survival world)
(The problem solved itself but re-appeared in 1.7.2)