How is it possible for someone to die of nothing?

My godfather tragically passed away in January leaving behind a wife and 5 children. He was a keen sportsman, lived healthly, enjoyed the odd treat here and there but so do we all! he was a fun loving person who always had a smile on his face, he loved his family, they were is life! But he suddenly dropped dead at the age of 44. A post morton was carried out but they failed to determine a cause of death.... He shouldn't have died! How is this possible!!! I need closure. Please help if you can.