My friends saved games disappeared and we're not sure why. He had just gotten onto MineCraft and went to single-player, and all his saved games were gone. He hadn't done anything to them and I'm wondering if theres anyway to get his worlds back.
I have the same exact problem. I use Dr.Zhark's Mo Creatures Mod. What I think happened was that while I was saving, it crashed and not all the chunks saved either. So I came back on to see my first world was missing. I opened it up again and saw I wasnt too far from my original area. All my creations were cut in half, while new land was formed over my mines.