If the other hen is not bothering her I think I would leave her be. It may stress her to be away from her friend.
I think she is finding comfort in the nesting box.
You can try giving her a soak in a warm epsom salts bath. This may be soothing to her.
If you can take her to a vet, they may...
I'm sorry sweetie, it looks like your hen is sick.
With the way she is sitting/standing - she could have an internal laying or reproductive disorder. It could be Peritonitis, tumors or cancer.
Try to make her comfortable and see if she will eat and drink. Hydration is important. You can try...
You can give her pedialyte if she is dehydrated. But not for more than a day or so.
Leave her be for the night and check on her in the morning.
If she has been like this for more than 48hrs, then it's most likely not an egg stuck in the vent. But feel of her abdomen to see if she feels...
It sounds like your hen is sick.
If you can post your photos - especially in the position that she is laying down in that may help.
How old is she?
Can you feel of her abdomen - is it hard, soft, fluid filled?
How many days has she acted like this? 2?
Do you know if she was vaccinated for...