pourly chicken

Hello Loukev and welcome to the forum.
I'm sorry to hear about your hen    I am not an expert but there are loads of people here who can give you good advice.  I would check a few things to help us help you.
Is her breathing noisy/rattley?
Is she poohing OK?
Does she have a temperature?  You can tell this by feeling under the wing of one of your other birds then using the same hand to feel under her wing to see if she feels hotter.
What is her crop like?
How is her comb?
Have your chickens been wormed?
In the meantime good luck and keep us posted