She MUST get some water in her ASAP.  She cannot survive without it.
If she were my hen, I'd give her at least 1-2ml (1-2cc) of water every hour or two.  When you're giving it to her, do it slowly a few drops at a time.  As for food, at her age (11 weeks), I'd have her on a Starter Feed, but some would argue that adult feed and baby feed is just marketing.  Having examined the labels of both, there is not much different in the two we buy, with the Starter Feed having a greater concentration of protein as I recall.
Wry Neck can manifest itself in different ways.  Consider this too:  she may not have been getting the nutrients she needed and that helped the onset of the head hanging.  The converse is also a possibility: her hanging head may have caused her to have difficulty eating or drinking and caused her to become weakened.  I know that sounds somewhat circular, but when chasing chicken ailments, we've got to consider causes and effects.
Anyway, you need to get water in her ASAP if she hasn't had water since yesterday!  She MUST stay hydrated.  I'd give her water with some vitamins and electrolytes too.  Feed stores have several different products that will work:  Rooster Booster products, Sav-A-Chic products, etc.  At 11 weeks, some need that boost of vitamins for proper development; others do just fine with starter feed and water.
I really sense that this might be a nutrition issue, so getting water into her with the vitamins AND electrolytes should help her to feel better quickly if we're on the right track.  Yes, do both at the same time.  Here's something you might not have realized:  if you treat for Wry Neck, the treatment will not hurt her.  It will only help her because Wry Neck treatment is primarily about getting higher dosages of vitamins into her to help her function and develop properly.
Let us know how the watering goes and how she responds.  Also, keep Eggcessive's post in mind too.  This could be Marek's as some of the symptoms align with that disease.  You might call the store from which you bought her and ask if the chicks they sell are vaccinated against Marek's.  Many hatcheries do that routinely, but you won't know until you ask...