Hi Roxanne,
First off, I would isolate her from your other chickens. I would bring her in the house and make her a nice place out of a cardboard box and old towels.
I've had some chicks and chickens I thought had colds or respiratory infections and for the most part cleared them up by giving them vitamins in their water, namely Poultry Cell. Something else that does nothing to cure them but might help them breath better is Vicks for chickens called VetRX. You would rub a little bit under each wing and near her nostrils.
I had one that these tricks didn't help, so I got an antibiotic, Amoxicillin, and gave her 125 mg twice a day for a week. She's a silkie so bigger chickens get twice that. I ordered that off of Chewy. It's called Aquamox. (I've heard other people call it Fish Mox.) That worked.
If you have an avian vet though, you may want to take her in.