Is she hot or cold, how's her poo, have you given her any sugar water or electrolytes. Do you have liquid poly-sol- vitamins on hand? If you do, try and give her a couple drops 3x/day. Do the other chicks let her eat?
She may need help in driking and eating if she's so weak but don't tilt her head back. She may need soft food, like her crumble wet down or and serve her that.
Let us know if there is anything else that seems wrong.