I just had this happen with my buff orpington. I brought her inside, gave her a cool bath, and laid her on a towel on the tile. She also was completely out. In a couple hours she was coming too and I helped her have some water. I eventually put her in a cage inside and provided eletrolytes in her water. I got that at the feed store. She took about 4 weeks to be ready to go outside and now she is out with the flock and doing fine. We had a 118 day last week, and she made it through that, I was surprised.
I also talked to a vet on a local radio show and he said it takes patience and time but if you are willing to work at it and take care of her inside she will be better. He explained that it takes time for her to grow new brain whatever, connections but that they can fully recover.
Here is a link to my threads, with pictures. I hope she gets better, but plan on having her inside for a few weeks.
https://www.backyardchickens.com/forum/viewtopic.php?id=518372
https://www.backyardchickens.com/forum/viewtopic.php?id=520337
I also talked to a vet on a local radio show and he said it takes patience and time but if you are willing to work at it and take care of her inside she will be better. He explained that it takes time for her to grow new brain whatever, connections but that they can fully recover.
Here is a link to my threads, with pictures. I hope she gets better, but plan on having her inside for a few weeks.

https://www.backyardchickens.com/forum/viewtopic.php?id=518372
https://www.backyardchickens.com/forum/viewtopic.php?id=520337