This may be coccidiosis, even though the chick is pretty young as cafarmgirl noted. You need to get some corid FAST, as it looks like the chick is very weak (normally coccidiosis isn't quite this fast, but the corid won't hurt). While you're at it, get some sevin dust, because if it isn't coccidiosis the chick could be weak from a mite infestation. Look closely around her face, you may see little red or dark mites. I lost 2 chicks a few weeks ago from a sudden mite outbreak, and they acted like this, weak and sleepy and then laying on their side. I had more chicks hatching, so I cleaned everything and dusted and sprayed, and when each chick hatched I gave them an extra dusting of sevin and they all survived. Get some water with corid into this chick right away, with a dropper if needed. Dust with sevin as well, keep the chick warm and try to get some nutrients into her. If she's weak from mites, she needs nourishment. Watery mash by dropper if necessary.
Hope she pulls through.