Isolate her and feed her yogurt with a few grains of salt, dissolved in warm water. Use a syringe; you don't even have to get it in her mouth, just wet her beak with the syringe and she will instinctively drink.
It's important to get her away from the others; baby chickens will peck a sick bird to death almost before you know they're sick. You may need to hold her pretty much constantly to keep her from panic peeping.
Edited to add: we all like to throw out 'I think my chick has XYZ' but the honest truth is they are frail little creatures and the reason a hen broods a whole clutch full of eggs is because not all are meant to survive. They seem to give up really easily and stop eating the minute they feel unwell, and can't survive for long without nutrition.