We had one chick that was like this out of our spring hatch. It got left behind in the nest because it was too weak to follow the hen and its siblings out. We took it to the vet fearing spraddle leg; but the vet said it was just weak because it was so newly hatched. We kept it warm, offered it mashed up hard boiled egg yolk (at the vet's suggestion), slipped it back under the broody that night and the next morning it was fine.
"Lucky" turned out to be a little cockerel, though (darn!) so we had to rehome him. He's three months old now and doing fine in his new home.