Start supplementing with Vitamin E and a bit of selenium right away. You can use human supplements if you can't find a poultry one. You can also give the E mixed in a bit of scrambled egg or tuna for the selenium. The deficiency is the vitamin E, the selenium is necessary for the body to uptake the vitamin E. Instructions here:
https://www.raising-happy-chickens.com/wry-neck.html
I would also get a human B complex tablet, and grind that up also, just to cover the bases. B deficiencies can also manifest with neurological symptoms. B's are water soluble and are not at risk of overdosing, excess will be excreted. You do have to be more careful with the E, too much of that can present a problem, see the link above.
It is not uncommon for one bird in a group to have the problem while the rest seem fine. That chick may have been deficient while developing in the egg, may have had less access to feed, etc. Also check your feed mill dates on your bag to make sure you have fresh feed (either stamped on the bag somewhere or on the sewn strip on the bottom of the bag), vitamins degrade with time.
If caught and treated early the bird may recover just fine and have no further problems.