Could you post a photo? How old is Ella? Chickens usually molt at 1-1 1/2 years of age, at which point they lose feathers and may become more lethargic, pale, and with shriveled combs/wattles. Once they are done molting, they will return to normal. To help speed up the molt, you can give high protein foods like wet cat food, mealworms, and other insects.
Mites and/or lice are another possibility. In that case, you can treat her by dusting with 5% Sevin dust or poultry dust, spraying with another poultry mite/lice spray, bathing her with flea and tick shampoo, or by applying some Frontline Flea and Tick control (its made for dogs and cats) to the skin under her vent. You might want to consider doing this to your entire flock, as if one bird has mites/lice, almost every bird usually does. It would also be a good idea to clean out the coop, spray it down with vinegar or a mite/lice spray, and then put in fresh bedding.
If not molting or external parasites, then worms are my next idea. I would purchase a good broad-spectrum wormer like Valbazen, SafeGuard, or the Worminator (it contains Flubenvet and is what I use). You can usually purchase those wormers from a livestock supply store, and the Worminator can be ordered online from here:
http://www.twincitypoultrysupplies.com/store/index.php?main_page=product_info&products_id=584
Other than that, I'm not exactly sure what could be wrong. If possible, I might consider isolating her so you can monitor her condition. Keep her warm and comfortable, and minimize stresses (loud noise, animals walking by, etc.). Perhaps offer some vitamins/electrolytes in her water, and encourage her to eat by feeding her favorite foods.
