Sorry to hear about your chicken.
I'm a complete newbie, but we lost a chicken this summer, our first year of having chickens. She was a Cochin, also so sweet. It was summertime and warm, but she never went inside at dusk as usual and everyone else did. We put her in, she could not hold onto the roost. We took her inside our house to keep an eye on her and in case it was an illness. She made it through the night, but was very weak and unable to stand up , so we took her to an avian vet. Cost a fortune. the result was a slight overgrowth of bacteria in her stool. Gave her anti-biotics for 5 days, but then she died. We think it was botulism, as she had all the symptoms. Everyone else was fine. Hopefully your other chickens will be fine too.