I had 1 that did that in a hatch 2-3 weeks ago. First of all he was a late hatcher and I had to help him because he pipped but didn't have enough strength to zip and the membrane stuck to him.
After he emerged from the shell he laid on his back and waved his legs in the air and flopped around inside the bator for 2 days.
I'd set him upright and he'd roll over on his back again. Every hour or so I'd stand him upright and hold him to help him get his balance. By the third day he could balance and walk but his head pointed straight up and he'd still fall over backward quite a bit.
I took him out of the bator and put him in a box with a few other special needs late hatchers. He still ran around with his head stuck straight up but was able to eat & drink somewhat. After about 5 days his stance was almost normal he could almost walk normal but still wasn't quite "right."
I took him outside and threw him into the brooder with his 40 other brothers & sisters and now I can't tell which one he is.
Important thing is, help him to stand up to learn his balance and strenthen his legs, and help him eat & drink because he can't see where the food & water is.
I don't think it's a vitamin deficiency because he started improving before I removed him from the bator and hadn't had feed or water yet. Once removed from the bator I didn't give him anything but regular water & chick starter. Feeding him Gro Jell vitamins like Fried Green Eggs did probably does help because problem chicks need all the help they can get. I'd say it's a neurological problem associated with being a late hatcher. Being mine was a late hatcher and the hatching & chick debris had been soaking in the water troughs for about 4 days and was rotten and stank to high hell maybe it was bacterial related.