My head hen was a fairly large girl and one of my first hens. She was given lots of treats, like an embarrassing amount. (I know better now) I went out to let the girls free range for about an hour before lock up and noticed "Fashion" looked to be struggling. She was extremely pale so I rushed her into my shop and placed her in a dog crate with towels to help keep her comfortable. Within 15 minutes she was gone. I suspect fatty liver. It was extremely fast because I had checked on them moments before letting them out and she was acting normal. I hated to lose my wonderful girl, Fashion, but I'm relieved it was fast. I do worry about her feeling unwell in the months and/or years before her death but I hope the worst of it was quick. Seems like a hard jolt in certain areas can also cause them to bleed out internally. Was she pale or did you notice anything like that? I'm so sorry for your loss.
Your flock will thank you.