Often times this is called sudden death syndrome. They are perfectly normal, and then dead. So discouraging. A dangerous time, is right after hatching 2-3 days you can loose them, and then right at 4 months - just when you think maybe it is time for eggs. So discouraging. An old time chicken keeper told me, that at that age, they are approaching full size. She felt that they had been hatched with a minor defect that was able to maintain the bird, until the bird got near full size.
Take a long hard look at the rest of you flock, if they seem bright eyed and bushy tailed, do not worry about it and go on. Once I lost one bird a month, 3 months in a row... just dead like that. I was getting very worried, and never had another one in years.
If these are hatchery birds, 10 to one, genetically speaking, the birds in your flock are not even related genetically. There is no way of knowing. If these are birds you hatched yourself, well, the healthy ones will go on to reproduce, so this should terminate.
If you would start to loose 2-3 birds this week, then I would have them necropsied, if you have that available. I would not bother if it was one bird, that was apparently healthy and then just gone. I would assume it was a genetic fault, and probably none of the others are that closely related.
Mrs K