I am so sorry for your loss. Chickens are masters at hiding their illnesses and keeping how bad they really feel from us. A chicken on death's door can get a burst of energy, then just lay down a few hours later and go to sleep.
Without a test, it's impossible to know what got your hen. If you lose another one, please place the body in the fridge (not the freezer) and contact your local vet's office to see about a necropsy. They will have information on where to send him or her for a thorough examination.
She could have been egg bound or had Egg Yolk Peritonitis. Did you ever seen evidence of runny, yolky poops?
If she was overburdened with internal parasites, that could give her a poopy butt and make her unable to function properly.
It's possible she got Coccidiosis (though unlikely in adult hens). Did you ever see anyone standing around hunched and puffed, bloody poops under the roost, or anyone going off their food?
It's also possible she had a stroke or some other neurological issue suddenly popped up.
Please don't blame yourself. As soon as you knew something was wrong, you took action and did what you could. She lived a happy life with you taking great care of her and loving her. Sometimes, they just go too quickly.
MrsB