My understanding is that some chicks are going to die. As sad as it is....it could be a random thing, a congenital defect, lots of things. IF more appear to be ill--- post back with symptoms.
Keep a close eye on them...but loosing a chick is, unfortunately, a pretty normal part of chick rearing according to what I have read on the subject. Sorry for you loss. Let's hope it is just that one.