part way through their 2nd laying round, and one of my girls, a rhode isLand red, has been un social, not eating, and hiding in a corner.
Hi
@Rachel Johnson 
Welcome To BYC
I'm sorry to hear that your hen is not doing well.
@21hens-incharge and
@aart have asked some great questions and made excellent suggestions.
I agree with bringing her in or placing her somewhere warm -especially if you are in the US with all this cold weather.
More information and photos/video are always helpful, so if you can answer some of the questions asked previously that would be great.
You mention she is part way through the 2nd laying round, so....is she somewhere around 2yrs old and has gone through her molt? I'm not that great with timelines, so correct me if wrong.
Check her crop to see if it's hard, soft, squishy or her breath is sour. Look through her feathers for any parasites. Also while you are examining her, feel the abdomen for any bloat/swelling or feeling of fluid and just to make sure carefully look for any bruising, cuts or other injuries that could be a contributing factor to her behavior.
Getting her hydrated if most important, so see if she will drink for you. After she is drinking, then offer food. If you have electrolytes or poultry vitamins those can be added to the water.
Let us know how she is doing.
I know that sounds like a lot, but unless you have a vet that can see her, what you have to do with chickens is eliminate possibilities, since there are so many things that can go wrong.