I fear nothing in the world, why is that? Has the heart died?

If the questioner means that he does not fear, for the sake of Allah, the blame of any blamer, in the sense that he fears none but Allah, then this is a description of Allah's prophets and His chosen ones. This is the fear of Allah alone, and whoever is like that has a living, sound heart. As for one who does not fear Allah and rebels against His commands, his heart is dead; he does not know his Lord and he stands with his desires, even if they incur Allah's displeasure.