Have you added any new birds to the flock recently? Sometimes it is hard to treat a sick chicken without ruling out a whole list of things. Has she had any history of eggs breaking inside, or laying shell-less eggs? Internal laying and egg yolk peritonitis is a common cause of illness in hens that age. I would check her crop for impaction or not emptying by early morning. Look for any signs of lice or mites on her skin, especially in front of the vent. What color are her droppings, and are they runny or watery? Does she have any paralysis in either leg or wing? Things like lead poisoning from eating old lead shot or paint chips, mold in feed, eating a toxin in dead animals (botulism) can cause lameness. I would feed her as long as she is interested in eating and drinking. A vet might be of help, or you could try a powerful antibiotic such as Baytril (enrofloxacin) for a chance that she has an infection. I hope that she can recover.