What can be the cause this? I've heard recently of 2 people this has happened to, one was aged mid 60's one 30 something, they had both been on a night out, and never woke the next morning.
That's how my uncle died. I'd been with him during the day and he was fine. Went to bed and never woke up. Unexplained death. No signs of heart attack, he hadn't been drinking. His heart just stopped.