I’m so sorry for your loss 

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Sorry for your loss. If you can keep her body cold in a cooler on ice or refrigerated until morning, you can take her in to your state vet lab for a necropsy. That is the best way to get a diagnosis. Here is a list of contacts for your state vet lab:
https://www.metzerfarms.com/poultry-labs.html
Same thing happened to my maran hei hei, he showed no symptoms of anything and dropped dead, then I found bloody poop and got corid but still lost 25% of my chickens the next dayView attachment 3894588
The necropsy determined that cause of death was most likely coccidiosis. We’ve seen no signs of bloody, foamy, or yellow stools. I picked up a bottle of Corid this morning and have begun a five day treatment in their water.
Argh. That’s awful.Same thing happened to my maran hei hei, he showed no symptoms of anything and dropped dead, then I found bloody poop and got corid but still lost 25% of my chickens the next day
I'm so sorry for your loss. I lost a sweet 8 week old chick yesterday and its heartbreakingA short while ago, I discovered one of my 12 week old Cuckoo Marans, Fifi, laying motionless at a funny angle under the henhouse inside the run. I retrieved her body -- no apparent injuries; her legs and wings were stiff, her eyes closed and slightly sunken, but her head and neck just flops around.
View attachment 3890918
She and her sisters had been acting completely normal yesterday -- it's very hot (100°F / 38C) today, but their coop is in the shade and they have plenty of cool, clean water and have been eating normally. They are on a 20% start and grow feed, with occasional (chicken-safe) scraps, including some fresh basil yesterday evening. The coop perimeter is intact and no signs of predator intrusion. I have a camera in the run, but of course, her body was almost directly underneath of it, in the only real blind spot.
View attachment 3890917
Is it possible that she could have broken her neck? My partner and I are just heartbroken -- Fifi and her sister, Freda, were the youngest of the flock, because we really wanted to have some Marans in the mix.