I was in the nether, looking over a lava sea. I had my new diamond pickaxe with me for mining glowstone. But, instead of pressing shift, I held caps lock.
I had something similar happen to me; I backed up one of my saves, and now it is gone from my saves folder. (I can still play it). Perhaps you accidentally removed level.dat from the folder? TRY Looking for it elsewhere in the minecraft folders.