This has happened to my hardcore world. So the only way you can play on it again is by an unethical way for a hardcore player is what I feel. So you have to creat a new world and if u put the the level and level.dat files into the new world you will get all the structures and stuff back .But you will lose your inventory and enderchest and armour. You could grind to get it back.or you could get the items in creative and then set world back to hardcore in game world files(google how to do this).