I was working in my hardcore world, when I had to get off. I saved the world and exited. The next day, I logged onto my PC and loaded up the launcher. I got into Minecraft, and looked for my hardcore world. It wasn't hard to notice that it was missing. I had about 5 total worlds, and had only named the one that got deleted. I named it hardcore. I looked through my saves folder in %appdata%\.minecraft, and it was not there. I had spent at least 2 hours, probably closer to 5, working on this world.