Chicken suddenly died

We have 3 dozen chickens (11 months old), Barred Rock, Americanus, Austrolaups, Rhode Island Reds and Comets. Everything had been going well until today.
One of the Americanus was lethargic and just sitting there in the coop this afternoon. We hadn't noticed her like this y-day!
When I picked her up she was nothing but bones!? Her eyelids were closed and she didn't move.
She passed this evening and I'm concerned about what caused this!?
The weather has turned cold (single digits at night). I have a heat lamp under the water bowl to keep their inside water from freezing.
They have access to plenty of food and water (changed daily) ... I give them plenty of cracked corn each morning.
We have 3 hanging feeders that have a mix of laying mash, cracked corn and laying crumbles. These get filled daily.
Egg production is good (26-32 eggs/day) and I haven't noticed any other chickens with any symptoms....yet!!
ANY IDEAS!??
Thanks... Thomas Clodfelter

There are any number of possibilities. Ideally, you should have done an autopsy on the bird as soon as you found it. Since this hen was emaciated something was likely preventing her from absorbing or digesting her food, given that she was eating (do you know this?) I remember one time having a "runt" turkey that just wouldn't grow and thrive like the rest from the same hatch. When I butchered it I discovered a syringe needle in the gizzard, which was crowded with scar tissue from where the needle had punctured from the inside many times. Sometimes an egg gets stuck in the egg duck and "jams" with the next egg coming down, but this usually means a quick death without malnutrition. These and other problems could be discovered by taking a look inside.

Alder,
Thanks for the reply!
I'm not sure what I would have been looking for unless it would have been something obvious like your syringe..!?
I was really surprised at how boney she was... I will try and look at a few others over the next few days...!?

Thanks...Tom

Impossible to say at this point, but daily observation will help leading up to situations like this, an animal doesn't lose that much body mass overnight, granted with a herd running around its not an easy task. We hand feed our free ranging birds every afternoon just for this purpose, we can count them and look at all of them for problems like this showing up. Also during your rounds watch for individuals showing signs of being sick, they'll generally stay away from the others staying in one location not doing the search and peck routine with their head/neck retracted close to the body, looks very similar to a cold chicken. We had a situation close to yours, we never found the exact cause, but highly suspected it was associated to rats, once we beat the rat numbers down, we stopped having sick/dieing chickens.

Curious have you checked all the others?

Not helpful for this time but if you do have any other unexplained deaths, a lot of colleges will do necropsies on deceased chickens. Many places will do them for free or a small fee. If you go over to backyardchickens and do a search for 'necropsy' there are a bunch of threads that talk about how to go about sending a chicken and sources that can help you find a place that will do it for you.

In case one passes before you are able to do the research, the most important thing to know is to put the chicken in the refrigerator (not freezer) until you are able to send them off.

And in the meantime-it might be a good idea to take some stool samples to the vet to check for parasite loads. Most vets will do fecal checks, even if they don't treat birds specifically and it usually costs between $5-15 each test. Many people would say to worm on a schedule but I don't believe in treating until I know there is a problem. Also, be wary that there will always be some parasites present, healthy animals always carry some load but can keep them in check. The vet won't tell you this so make sure to ask how many parasites are present and do some research to decide if you want to treat.