It at all possible, please get that bird to an avian vet pronto. She needs help emergently.
Also, give 3 drops liquid Polyvisol childrens vitamins (without added iron) daily - dribble gently along her beak line with dropper, so she swallows on her own and doesn't choke. This is not the solution to the problem but hopefully will give her a bit of strength to hang on while this is being sorted out.
She may not be eggbound or internally laying, given that you got an egg from her yesterday (sure it ws from her?) but she may well need meds quickly to get through this.
What is their diet?
Has she lost weight?
Have they been wormed?
Have you ever seen mites on any of them?
Any new birds introduced to the flock lately?
Any other insights?
JJ