Hen very sick with sour crop

[Quacking ducks]

Yesterday I noticed that one of my hens (Sprinkles) wasn’t acting right. when I picked her up, I noticed that she has sour crop and a very bad smell coming from her mouth. she is very underweight and today she won’t drink water or eat the plain yogurt I give her. Her comb is purple and she is acting very lethargic. did I find her too late or can she still get better? I have her isolated in her kennel with water and a little bit of plain yogurt. What else can I do? Thanks