
I'm afraid the only way you would know for sure would be a full post mortem. I have heard various theories for sudden death, and it being likened to Cot death, one theory being a sudden surge of electrical activity that causes like a short circuit in the brain.
I do think it unfortunate that your breeder has not replied to you, have you tried sending a letter recorded delivery? Even if they can shed no light on what may have happened I would imagine they would at least want to sympathise with your loss, if they are a reputable breeder.