Hard to say what happened but it is possible the broody (or someone else) picked the feather fuzz off the chick's head. Perhaps there is something wrong with the chick that you can't see?
Edited to add: I missed the comment about "spray the coop" earlier. What are you spraying for? Could the chick have been attacked by mites?
No spraying with water cuz its very hot. We spray their dirt baths when it gets very hot.
Keep us posted.. How long had it been since you had seen her?
e She was in a wire dog kennel for three days, in the deep shade of a tree of course, and she could reach the ground to scratch in and cool off. I was amazed that she had sisters around her at all times (except at night when I closed them in their coop). When everyone was scratching through the compost bins there was at least one sister hen that stayed behind to keep her company. Such good sisters!

