Do you have a place where you can put her inside your house or garage with a bit of heat? Heat lamp or heating pad or even a big towel warmed in your clothes dryer will help warm her up. If she's sick and not eating, she can't regulate her body temp.
Next, give her a bit of sugar water to elevate her glucose. This will revive her enough to get her to eat something which can help her regulate her body temp.
Next we need to figure out if this is a bacterial infection or a stuck egg. Has she started laying? When was the last egg? Was it soft, thin-shell? Check her vent for signs of swelling and pulsating heavily. Is there a runny discharge coming out? Are her butt feathers very wet?
What was she doing prior to your noticing she is sick? Is there any compost bins where you dispose of moldy spoiled food? Have you checked the feed to be sure it's not moldy? Smell it. Does it smell "off"?