Minecraft: Didn't Save & Close and now won't open [duplicate]
Solution 1:
It seems like the file is corrupted. You can still see the file because it still exists, but you can't access it. First of all, back up your files often, so this doesn't happen. Within the saves folder, copy the save, and rename it to "New World". Then, go into Minecraft and create a new world called new world. It won't completely be fixed, but it should recover some of the data.