How long has she been like this? Could you take her to a veterinarian? You can post on the "emergencies, diseases, injuries, cures," thread in detail. Maybe she is too hot on the heating pad.
I don't know if you live in a cold climate but ordinarily chicks would be fully feathered out by 6-8 weeks and not need supplementary heat .Have you tried giving her Polyvisol infant vitamins (NO IRON), 2 drops 3 times a day. This chick is going to pass very soon, if she doesn't get a diagnosis and proper treatment.
What are you feeding her? She may like warmed oatmeal with some plain vogurt, finely mashed hard boiled egg yolk, moistened a bit with yoqurt.
Does she have any symptoms like coughing, bloody stools? Have you checked her over well for mites or lice, they can quickly debilitate young chicks. Does she have worms - they can kill quickly when there is an overload.