Trying to understand the Death of our Husky?

I'm really sorry. Without a necropsy (autopsy after death) there is no way of telling the cause. Without any symptoms other than the lethargy, it's impossible to say. The breeder should be told, however. If it is some inherited defect, the breeder should be made aware. And maybe the breeder has some insight that would put your mind at ease about the male you have, or let you know if there's any cause for concern.