Greetings and
Can you describe the 'red neck'?
Sounds to me like maybe a bite from a snake, insect, or similar, maybe a reaction to a plant. Can you post a picture?
Either way it sounds like severe toxicity. The ruffled feathers indicate how unwell it feels.
It's smart for the chook to not eat or drink initially as this allows the immune system to properly devote all its resources to healing rather than digestion, but that said, some chooks take it too far. Suggestions for what to do to help the chook depend on how closely we can identify what is wrong.
If you know the chook has had access to dangerous plants or items to eat, activated charcoal would be one good idea. For general toxicity, about 1000mgs of Vitamin C would be helpful too, it can destroy toxins in the bloodstream after being absorbed, everything from snake venom to plant toxins and artificial chemicals.
But what you should ideally do depends on what's wrong. Some would flush the chook's system ASAP with something like bicarb of soda or epsom salts, but that red neck suggests the problem is likely not due to having ingested something wrong... Unless of course we're talking about the chook having swallowed a spider and then been bitten on the inside of the neck.
Depending on what the 'red neck' looks like, it may be an infectious disease you're dealing with; vitamin C will help with that too but, fair warning, this sounds pretty serious as you no doubt know, we may not be able to help your chook.
I'd separate this one from the others just in case, probably. Hard to offer any decent suggestions without a better idea of what's wrong though. A pic would possibly help that.
You may want to look up Marek's disease sometime, but it's probably not a priority right now as your chook sounds like it has severe toxicity to me... I don't think your chook has Marek's at the moment necessarily, or if so then it's not its main problem right now (pending finding out what that red neck is all about), but you need to know about it anyway as it's so common and has many similar symptoms to what you're describing. Also, they can have multiple things wrong with them at any time.
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Best wishes.