Internal laying can go on for years. Or it can kill in just one week of ovulation. Last summer I lost a one-year old Blue Andelusian, and she went from free-ranging normally all morning to stepping inside the run at noon and falling over dead. She happened to fall right on top of two of my oldest hens who were resting on some feed sacks beside the pop hole.
That youngster isn't the necropsy hen in my photos at the above link. That was an older hen who had been acting lethargic for a while, but it wasn't anything I could diagnose. Judging from the hardboiled eggs, they had to have been cooking for weeks or months inside her.
The symptoms also fit a reproductive disease such as cancer or a bacterial infection such as salpingitis, neither being treatable.