Welcome To BYC
I'm sorry about your chick.
I'd try to get her to take drops of sugar water or electrolytes, see if that helps perk her up. If you have Poultry Nutri-Drench, give her 2-3drops twice a day, in addition to getting fluids into her.
Also offer mushy chick starter after she's drinking fairly well.
Could she have gotten overheated or trampled by other chicks? What does her crop feel like?
If you have photos of her, her poop and your brooder setup, those are always welcome.