I am fairly new to chickens, but not birds so take this with a grain of salt and of course keep looking for what is wrong until it makes sense that you have found it.
I am guessing your bird died of some type of anemia (lack of red blood cells) - this could be caused by many things but the first things that come to my mind is mites or lice that are so heavy that they literally sucked the blood out of your chicken - a little at a time...
Some mites / lice only come out at night so you don't see them during the day (like vampires!)...if you don't already do this I would get some food grade DE (Diatomaceous Earth) and carefully dust all your birds and strip the coop and nest boxes and dust them all as well - especially perches or anywhere there are cracks in the wood for them to hide. The DE will kill the mites / lice as they have soft bodies but will not harm your chickens or you.
It is a very fine powder so do be careful not to breathe in a lot of the dust (hence why you carefully put it on so as not to create a lot of dust with it).....
My first hens started losing feathers and turning pale and they had mites / lice...then I dusted and now they all look really good (except they are molting but that is typical in the fall). You can sprinkle a little bit of it in some mash for them to eat as well, that will kill internal parasites as well and even people will eat DE for worming.
Like most things that work well, don't over do it, but do get enough on them (I carefully rubbed into their feather to get it on their skin where the bugs were) so that it works, doing the coop where they live, sleep, eat, lay eggs, etc. is also a must and then put in new bedding. I didn't replace the bedding when I did it - I just moved it to one side and put the DE down then moved it over and did the other side, but then I only had 3 hens in a small chicken tractor but it still worked.
If that doesn't work, it is possible that chicken had something else wrong with it and it will be the only one to get sick / die...or you may have something else wrong with management, diet, etc. so keep reading and trying things until it gets better....
If you chickens are all molting right now (I am in Oregon and all my chickens are molting now) and only 1 out of 4 adult hens is laying at all, but they will start again when done molting.
Good luck !
Anne