I'm so sorry to hear that you lost your girl.
I would assume she got into something harmful, toxic, or poisonous... For it to be so sudden like that, there had to be a catalyst. And she was acting perfectly normal the day before? Chickens can handle sub-zero temps very well (minus frostbite on toes and combs), so I doubt it was the cold that got her.
Did she have access to an area with shiny metal objects, anything that's painted, or any place you keep pesticides/herbicides/pest poisons?
Did you see her convulse at any point? Did you check her for wounds (snake bite, spider bite, injuries, etc)? What kind of substance did she vomit - water, foam, food? Did she do that on her own, or were you helping her regurgitate?
I would keep an eye on your other chickens' behavior and check the area where you keep them for any potential environmental toxins, INCLUDING PLANTS. There are some backyard plants that can be VERY toxic to chickens if they have access.
It may be a good idea, as @hennible suggested, to get a necropsy if you can, or perform your own (I would especially check her gizzard and stomach to see what she ate). When a chicken passes mysteriously and suddenly, it is ALWAYS a good idea to identify the cause if possible.
I am grieved for you. I hope this is an isolated incident and your remaining flock is unaffected.
MrsB
