Sorry to hear about your rabbits.
6 weeks old is very young, rabbit shouldn't leave their mother until they are at least 8 weeks old.
Was the rabbit vaccinated as it could be VHD (Viral Hemorrhagic Disease) as that kills very quickly and is very contagious.
The symptoms are loss of appetite, lethargy, fever, spasms and sudden death. Rabbits don't always show all the symptoms and they usually die within 48 hours of coming into contact with the disease if they're not vaccinated.