My chickens ill, what can I do?!

Hi,

We have five chickens and lately one of them (she looks like a leghorn, but i wasnt with my dad when he bought her so im not so sure) has become lethargic and isn't eating or drinking. We have checked and covered them all for mites and worms, and have checked her again for them.
She was out in the rain yesterday, and wasn't moving, which is when we noticed she was lethargic. I took her into the chicken coop, and she's not moved since when I put her in there, and all the other chickens are fine and out in the sun/wind.
I moved her into a cat travel basket and she's now in my house, I'm having to force her some water, because I put it there but she just isn't interested.
Her chest bone is REALLY promenant, and she can hardly stand up.. When she does it's like a penguin standing. Her comb is going purple on the ends, and she just will not eat. :/
As I said, I am making her drink water (though I am not sure on how much, so when she finishes this little square, how much should I give her?)
I tried giving her corn, and when she noticed it, she tried to eat it, but couldnt peck at it enough, we don't have any watermelon, or grapes etc. she won't even eat bread.

Can somebody please help me, I love my chicken and I don't know what's wrong or what I can do.

Thank you so much in advance!

Katonah.

 

Thank you for all the warm welcomes, I posted over in that area and got a hasty reply. She seems to be eating now, thanks guys. I hope she gets better.

Katonah.