Have you checked for signs of mites or lice? Severe mite overload can cause anemia, loss of appetite, and the eventual inability to walk due to weakness. I had it happen to one of my birds
I'd follow this route of exploration that humblehillsfarm offered above before trying to chase down Mareks. Mites and lice are a common problem with chickens of all ages.
She could also be dealing with worms of any number of variety, and they will sap the strength of the healthiest of birds in short order. Sadly, the birds hide it quite well until you find one like you've found yours.
Has she or the rest of your flock ever had, or been treated for, intestinal parasites (Round Worm, Tapeworms, Capillary Worms, Gapeworms, etc)? When?
What does this hen's stool look like?
Where are you located in the world?
What kind of temperatures have you been having?
Has it been overly rainy in recent weeks?
How does her crop feel? Is it emptying overnight?
How long has she been like this? If you don't know, how long ago did you discover her like this?