My rabbit is giving birth and one was born dead how long does it last and what to do if it happens again massage them to breathe?

Let mom take care of her babies! If one is born dead, she'll know what to do. Don't massage the kits, try not to touch them at all. Consult a vet if you'd really like to. But just sit back, don't bother her or the kits, and just let Nature take course.