A healthy heart just stops?!
Yes. In the case of our son, due to an identified, but unexplained, electrolyte imbalance. That's exactly correct.
The irony--and, for us, the joy--of it is that, among our son's donated and transplanted organs was his (very healthy) heart. The little girl who got his heart graduated from college this year, and our family was delighted to attend--and joyfully celebrate--that occasion.