@Wyorp Rock @Eggcessive @azygous
I dont' have to much experience with this specific issue, but the above members will hopefully be able to help more.
I'm sorry about your hen, if it were me, the first thing I would do is start tube feeding. If she isn't eating, no matter what it is she isn't going to make it. Tube feeding sounds intimidating, but it isn't as bad as it seems once you figure it out.
This article gives good info on tube feeding-
Article 'Crop Feeding'
When was the last time your lady laid an egg? How does her abdomen feel? Squishy, doughy, hard, enlarged? Can you weigh her? Photos of her poop may help, green is usually from lack of food, but it might help other members. What do the bottoms of her feet look like? Any signs of bumble foot?
I hope the other members can take some answers of these questions and help you. I also hope we can figure out what is wrong with your lady. Like mentioned above, ISA browns are high production breeds, and sadly don't live too long. Your lucky to get them past 5, I'm so so impressed that you got her to this age, your odiously a great chicken momma.

I'd start with crop/tube feeding and go from there. Getting food in her is going to be most important at this point.