It would help if we could have some more information:
How old is your hen?
How long has she been acting like this?
When was the last time she laid an egg? (and was it a normal egg or not?)
How does she react when you try to touch her?
What happens if you take her off the nest and place her somewhere else?
What does her comb look like? (pale/red, floppy/normal)?
Have there been any changes recently? (bedding, feed, environment, new birds added....)
The more we know, the more we can help you identify what is wrong with your girl.
She is 3-5 years, I think she layed up to about 5 months ago and I think it was double yolkers,her comb is hard to tell as it is damaged,she only got bad a few hours ago when it started raining but she was a bit ill for about 3 days
Have you examined her thoroughly? Check for swelling particularly in her abdomen and compare with a healthy chicken. Does her breath smell bad?
The fact that she used to lay double yolkers and now doesn't lay, suggests that it might be a reproductive issue. Perhaps internal laying which can lead to an infection.. Unfortunately production birds are prone to problems like that. It may be that it is putting pressure on a nerve causing her to have problems walking.
Supporting her in an upright position with the aid of a chicken hammock or sling (which can easily be made from items around the house) may be beneficial.to her care (ie make her more comfortable) but unfortunately if she is laying internally, the long term prognosis is not good. If she has an infection, then here in the UK it would mean a trip to the vets for antibiotics or if she didn't improve on her own, then cull. I know that in the USA you are able to buy antibiotics without a veterinary prescription but without knowing what the problem is, you should not administer anti bs "just in case".
Good nutrition is important to support a strong immune system so encouraging her to eat is important. Try soaking her pellets in water and mixing yoghurt into it to make a porridge consistency mash or make her some porridge with added yoghurt ... most chickens go wild for it.. Also soaking bread in milk or yoghurt will give it more nutritional value and ensure that she gets plenty of fluids and you could treat her with some scrambled egg or tuna if you haven't tried those.
You may also want to research ways to euthanize her should that become appropriate. Having knowledge before the event becomes necessary, is better than last minute panic about how to do.it when a bird is clearly suffering.
Sorry I can't offer a definitive diagnosis or way to treat your sick hen.... unfortunately hens often don't have a long lifespan because of the strain on their bodies due to being selectively bred for maximum production.