I wish I could tell you to do X Y Z and she will be fine. Unfortunately it's not nearly that simple.
Sometimes they can injure themselves jumping off a roost if it is high, if they are a heavier breed, and/or if there isn't room to "fly" down. I don't really know what your situation is, so I'll tell you about mine. I have what are called large fowl, meaning not bantams, probably around 4 or 5 lbs, maybe heavier. Their roost is 30' above the floor. There is about 6' to 8' between the roost and the wall, so that they can flap several times on their way down, sort of flying half sideways. 30" might be too high if they had to jump straight down without being able to cushion the landing by flapping their wings. I would put a ramp there if there weren't enough room, but sometimes chickens will jump down when they really shouldn't.
The cracked corn isn't as nutritious as the crumbles or pellets. They do love it, though. Sometimes if you mix them all together, they will eat the corn and leave the rest, which means they aren't getting all the nourishment they need. Best to give the corn as a treat by hand and separately, and not more than 10% or so of their food, especially if they don't have forage available. Where I live, some people still call cracked corn "chicken feed" and don't buy pellets or crumbles. Years ago, before all this was researched as it has been now, chickens would just be given corn and table scraps, plus they were free to forage for whatever bugs and plants they wanted to eat.
None of which really answers your question, because I'm sorry to say, I don't know. If she has injured a leg or foot, she might start eating and drinking on her own in a day or two. If you have a place you can separate her and let her rest, and it is a sprain or the like, it will give her a chance to rest the injury, if there is one. It's best to encourage fluids bu dripping them along the side fo their beak and letting them pull the drop into their mouth. This avoids squirting the water into their lungs. But in the end, she will need to eat on her own to survive. You can try offering a small amount of crumble or pellet mixed with a bit of water, as they usually like this better, til she starts eating on her own.
She could have any of several other problems, though. She could have a tumor or be an internal layer. She could have eaten something that has made her ill, especially if they have access to something like your compost pile. Things can happen in a protected run, too, of course. A neighbor could have put out warfarin type rat poison, and a poisoned mouse could have gotten into your run and she ate it, for example. She could have heart or liver disease, or some other serious condition.
If the others show any sign of not feeling well, of course, that changes things. If you bought them as adults, perhaps you could find out whether they have ever been wormed. Also, if you haven't already, you need to check them closely for lice and mites. If they are hard to catch, do this at night, as they are much more docile when you pick them up off the roost. You'll need a helper with a good light, of course. A really heavy load of either worms or lice/mites can actually kill them.
Also, be sure to check the bottom of her feet for bumblefoot. This happens when a staph infection develops in a foot, often the soft center pad, after a puncture wound. I've never dealt with it, but you can do a search here and find lots of threads about it.
Chickens are very good at hiding that they are sick. When they reach the point that they don't want to eat or drink, things are usually very serious. It may be that you won't be able to help this girl, and won't know why not. So, onle last thing. If you are in the US, you might check with your state vet office to see about having a necropsy done. If you happen to be in California, this is free. Usually it is not expensive.
I'll give you some links for more information about some of these things. I hope this turns out well for you and your hen.
http://ohioline.osu.edu/vme-fact/0018.html
https://www.backyardchickens.com/t/...ll-seeing-live-worms-in-poo/0_20#post_9315842
https://www.backyardchickens.com/t/...perazine-dosage-for-bantams/0_20#post_2525157
https://www.backyardchickens.com/t/40141/link-find-your-state-vet-for-a-necropsy/0_20
