Lots more info needed before anyone will be able to help you out here.
What age, what do you feed your flock, male/female, any recent changes to flock or diet, have they been tested for worms, is the chicken's crop empty in the mornings first thing?
you will need to try get them to drink first and foremost and to eat. When did they stop eating?drinking?
What is their poop like?
If you could answer as many questions as possible it will really help. Also photos of said chicken and poop if possible please!
Thank you for the photo and details.
She doesn't look good IMO. Do you have any poultry vitamins on hand? It looks like she would need something right away. Can you bring her inside or somewhere warm? Do you know when she last laid?