We've had this happen twice in recent years. About 2 years ago my GF lost a bird that seems healthy and was laying eggs. She came out one day and it was face down in the coop. This spring we had a 3 year old Welsummer hen who was the Alpha hen of our flock at the time (she was kind of a Terror personalilty wise though) who was walking around in the yard looking fine, it was a beautiful spring day and I was cleaning up the backyard. An hour later I went to refill the food in the coop and she was laying on her side dead! It was quite a shock as she was a big robust hen with lots of energy. So I think sometimes it just happens. No other hens got sick or died during the passing of either of these birds. Sorry for your loss.
