Minecraft worlds are not saving what is being built. It saves the position you were in and the blocks/items in your inventory, but not what's being built. I've tried uninstalling and reinstalling minecraft, thinking it would fix the problem. Now only half the time it would save. After that, it stopped saving at all. Before the bug happened, MineCraft would occasionally crash and would be out of memory. Since the bug, it has not done either. I've tried moving the game directory to C:\minecraft but that didn't help anything.