The same things that happen to the rest of someones money when they die. Bitcoins aren't magic.Most people leave a will. That clearly sets out where all your property goes when you die.If you don't have a will, the following people will receive your money. All of it goes to the top person, unless they die, then it all goes to the next person down, and so on:
Your spouse/children (50/50 split, then split among children).
Parents (split between them).
Siblings (split among them).
Grandparents (split 50/50 between both sets of grandparents).
The government gets your money if you have no living relatives or a will.