All of your hens may die, illnesses in chickens is very common and kills quickly. Everything from mites to respiratory diseases to cannabilism.
I would first check for mites, look at the eggs if you have any, for tiny red spots like blood droplets. Then check your laying boxes for more tiny red blood droplets, if they have mite infestations it could kill your birds and keep them from laying. A chicken is capable of stopping egg production on her own, weird I know, but they can if the laying boxes are infested.
Also, being immature birds still if you are feeding them adult feed with high protein content it will destroy their kidneys in a matter of one or two days. Go back to chick grower or just cracked corn. If it's a respiratory disease very common in birds, I'm sorry, they will all likely die off.
at 10:05 AM on May. 21, 2009