It appears that she has been attacked by a predator or pecked and the skin is gone over her lower belly. Was her bottom bare of feathers earlier? Chickens can vent peck each other to the point of cannibalism sometimes. I haven't seen one look this way in pictures before. She will need to be separated from the other chickens if you do not put her down. She will need to have her bottom soaked in warm water with betadine or chlorhexidene, or a mild soap daily, and kept clean. Then between baths I would apply neosporin/triple antibiotic ointment to her wound. Vetericyn wound spray is also good to use. The skin will need to fill in over a couple of weeks. So sorry this happened. If she is suffering, it may be best to put her down, but chickens can mend from some pretty bad injuries. Give her some chopped egg, and make sure that she can reach her food and water.