Hunched ill hen

Came into the coup this morning and found one of my chickens not really moving and very hunched up. I have isolated her and giving her water with a tiny bit of tonic in, and a bowl of food - pellets, a few milie worms and brussel sprout leaf chopped up - I can't see any cut or wound. I have put her in my cats travel basket at the moment it is on our garden table at the top of the garden should I bring it inside so she is not cold ? Although weather is not that cold anyway here ? this is the first time I've had an ill chicken so slightly worried about what to do for the best. And wondering what could be wrong with her ? She's just very hunched and not moving much and shutting her eyes a lot there is no discharge from her eyes or mouth ? any tips on helping her recover or what could possibly be wrong or what I should do greatly appreciated

You might find some pointers here;
https://cluckin.net/the-sick-chicken-symptom-checker.html

I no longer keep chickens, I used to have a small flock of 24 Rhode Island Reds, many, many years ago! I remember one of mine doing this, turned out she had pneumonia.
Hope yours recovers.
Thinking of getting a couple of Leghorns in the future.

Br.