You said that she was 6 years old, so internal laying and cancer are common. Has she lost weight in her breast muscles, or is she full in her lower belly? How does her crop feel—empty, full, puffy? Cold weather in winter will push some over the edge. I have lost 4 older hens 4-6 years old since Christmas. All had ascites or signs of internal laying.
You can get a feeding tube from a vet or a nurse friend, plus syringes at tfeed stores. I made one out of aquarium air tubing (from Walmart) by cutting a piece about 15 inches long, and burning athe end that goes into the throat with a lighter to soften sharp edges. They will fit a regular tipped 35 ml syringe. Feeding tubes from the vet will fit a 50 ml catheter tip syringe. Google Go Team Tube Feeding, a good thread to learn. You can grind up chicken feed and thin with water, or use KayTee baby bird feed. Fluids are most important at first, then food.