Sudden loss is hard, my condolences. I lost one about three weeks ago and I still haven't gotten over both the suddenness and sadness of it. I also keep reading trying to find a scientific answer. My hen was a black sex link not quite 2 years old, and had been happy and scratching in the yard an hour before we found her gone. I couldn't believe it. The most frequent response to my post was that it happens, and it might have been a heart attack. Sad, and I still miss her but also have two other chickens to care for. I'd like to prevent this sort of thing from happening again, so I think understanding is key.