You have to find out what's wrong with her before you can make her eat. Sounds a little like coccidia. You can get meds for it from your local feed store and mix it with her water and drip the water on her beak or stick her beak down into the water to get her to drink it. Keep her off the ground outside - that's where cocci likes to live.
If you feel as if you must intervene . . .
There's actually a feeding tube you can buy from a vet (one that treats birds anyway). I had to take my hen to the vet becuase she was eggbound for 2 weeks and refused food and water. He got the egg out but gave food to feed mixed with water and gave me a lesson on how to feed her like that until she got up the strength to want food for herself again. It's a long, curved metal tube with a ball on the end of it (about 4-5 inches long) and it hooks to a syringe (kind of like a needle). The food he gave was powdered and high in protein and calcium and you mix it with warm water until it looks digested (kind of runny, but not thick).
Get someone to hold the chicken down (press her belly to the floor, feet flat on the ground under her, wings against the body). You will do the rest. There will be two passageways in her throat. One (directly in the center) is the trachea - you can see the flaps of skin moving from her breathing. To the RIGHT of the trachea (the CHICKEN'S right, not your right) is the passage to her crop. YOU MUST GET IT ALL THE WAY DOWN IN HER CROP!! [EDIT: you can tell it's in the crop if you move the tube around while it's down her throat and palpate it out from her body, kind of like making her boob more pointy. The walls of her crop are be very flexible] If it goes into the trachea, the bird will die. Hold her head with her beak pointing straight up (thumb and index finger on either side of her head, under eyes and against her beak. Let the back of her head rest on you palm or the side of your hand.) Draw the food mixure up in the syringe and stick it down the hatch.
Chickens do not have a gag reflex, so don't worry about the act of force feeding being uncomfortable to her. Any kind of thrashing will be her just not liking it. After the feeding, she will instantly have a full stomach. Careful how much you give her though, overfeeding or water-logging can be deadly as well.
And btw my hen is doing much better and she's put on a good bit of weight but now she is spoiled and only wants me to feed her buscuits . . .
