It is impossible for the poultry backyard owner (or even the local vet!) to diagnose what might be ailing your fowl. Many poultry diseases have similar symptoms  no matter the illness. It is extremely difficult to pinpoint exactly the disease or ailment.
Some of the typical symptoms exhibited by a sick chicken include loss of appetite, reluctance to move about, fluffed up feathers, lameness, drooping wings, feather loss, lethargy, dull eye, respiratory problems, diarrhoea and soiled feathers around the vent.
We tend to surmise that dirty bottoms and diarrhoea are a result of worms. Lameness in birds is due to marek’s disease and that a sudden death is exactly that. The only way to be absolutely sure would be to examine and post mortem the deceased bird and submit the tissue samples for laboratory analysis.
Unless you know what ailment is causing your chook to be crook it is always good practice to isolate it from the rest of the flock. The best place is a warm, dry area (preferably a wire cage or even a cat cage) with fresh bedding, water and feed. Once isolated you can monitor your hen and see if she is going to overcome her ailment ...or not.