After seeing her age and her runny poop, I would have thought it was probably coccidiosis, which causes extreme weakness and dehydration. The drooped neck can be a sign of dehydration, Mareks, and other illnesses. I have doubts about Mareks, but Rebascora is a very good authority about Mareks, and she has much experience. It has many different signs in each bird, and commonly is found in birds between 4 and 28 weeks old, but can happen to older birds as well.
Refrigerate her body, and contact your state vet to get a necropsy and testing. It is very worth knowing what caused her illness. If you see other chicks with diarrhea or sleepiness, go get some Corid (amprollium) from your feed store, and start treatment for possible coccidiosis.
One person mention possible Newcastles, and it is not a disease found in the US, at least the exotic newcastles that is seeious. Another mentioned sulfa drugs for coccidiosis, and those are only avilable with a prescription from a vet in the US.
Please let us know what is found with the necropsy, and so sorry for your loss.
