I am so sorry. That is always so difficult.
We recently lost a hen as well. She was a two year-old leghorn named Otis and though we have around 50 chickens, I still felt sad for her. We believe she is the leghorn who got a prolapse and because of this got an infection inside of her. She has had it for some time but wasn't showing any symptoms. She passed in her sleep while on her perch.
Hens over one or two years of age will often die from problems linked with laying such as internal laying, water belly, infection or becoming egg bound.
I wish the rest of your flock the best of health.