Quiet Hen: Sick?

There are a lot of possibilities, especially at her age. Are they on a regular deworming program? Any evidence of lice/mites? Any new birds brought in recently that might have brought in some new strains of coccidia? How is her body condition, does she feel thin? Any swelling in her lower body like retained fluid? Basically this could be anything from internal/external parasites, some kind of infection, reproductive tract problems like internal laying/peritonitis or some kind of cancer. So you just have to start with a thorough physical exam, treat for the most obvious possibilities to rule them out and go from there.