Well, first remember that if you hatched these babies at home, chicks don't actually need to eat or drink for the first two or three days... so the fact that your home-hatched chicks aren't eating or drinking immediately isn't always a cause for concern. Strange but true. Hatching is hard work, and with the yolk that they have just absorbed to sustain them, sometimes they just take time to rest and recover.
That said, if you've had chicks shipped rather than hatching them at home---or if your home-hatched chicks are a couple days old or seem weak---they will definitely need to eat by the time they get to you. If they are not eating or drinking, you'll need to take action.
When you have genuinely weak chicks, there are a few things you can try to help them recover. First, make absolutely certain that the babies are warm enough--weak chicks may not be able to find their way to the warm, 95 degree side of your brooder! Next, check for pasting. Pasting is what it's called when droppings have dried around your chick's vent, sealing it shut. Obviously, if your chick is unable to have a bowel movement, it's a serious condition, and the pasting needs to be removed immediately! Keep in mind that having some droppings on your chick's down, while unsightly, is not pasting, and the chick will eventually preen it out--you needn't intervene in that case. It's only when the chick's vent is sealed shut that there is an immediate danger.
To remove stubborn pasting, you can use a warm, wet paper towel to soak the droppings off. BE SURE that the chick doesn't get too cold during this process; chicks that get chilled are more likely to have loose movements, which are more likely to get stuck in down and cause problems. Pasting can become a vicious circle if the chick gets too cold during the process of removal.
If the chicks are warm enough and the problem isn't pasting, you can try dribbling a few drops of sugar water or a molasses solution alongside their beaks. These empty calories can sometimes help give them enough energy to eat and drink on their own in earnest. This is not good long term, of course, as sugar has no nutrition to speak of, and too much sugar can give them diarrhea, which could cause pasting and associated problems. However, the hydration and the burst of energy from the sugar can sometimes help them get over the hump.
If they are eating on their own, but are just a little unsteady, you can also try warmed, plain yogurt mixed with their food. Believe it or not, finely chopped, hard boiled eggs (or scrambled eggs) can also help, and is high in the nutrition they need. Again, these are not good long term foods, but can help a chick that is stressed from the hatch or from a rough journey. If the stressed chicks are getting picked on, it's usually best to separate them from the others, perhaps with a bit of chicken wire, so they don't have to compete with the others and have the chance to recover. Remember, they will all need appropriate warmth, food, and water; and they'll generally feel better if they can still see your other chicks. Since they are flock animals, they want company and don't do as well when alone.
Last, please remember that these techniques can help chicks that are stressed and just need a little TLC to help them recover from a stressful hatch or journey. If you think your chick is ill with something else, you'll need to get her to a vet for a firm diagnosis and treatment options.