Chickens should be wormed twice a year as a preventative measure. There are many different chicken wormers on the market. You'll want to pick a multi wormer and alternate with a different multi wormer the next time. This prevents the chance of parasites becoming immune to any particular wormer. Ask your local feed & farm store for help in choosing a wormer, they'll know which ones work best for birds in your area.
When chickens don't feel well they are very good at hiding it. Some signs of an ill chicken include puffing up, with slightly ruffled feathers. They will stay to themselves & often stand or sit facing a corner as if they are being antisocial. What's wrong could be any number of things. First separate the ill chicken from the rest of the flock In case it is contagious. In doing this you can also evaluate whether she is eating & drinking.
Check the chicken over for any visible signs of injury or infection, check the vent for any abnormality, check feet, legs & toes for any problems or swelling.
If you don't have a vet that treats chickens you'll have to do the best you can to care for her yourself. My favorite place for advice on treatment for a sick chicken is Lisa Steele's website. I am a huge fan of her educated natural advice on chicken health issues. You can find her advice on treating injured or sick chickens HERE scroll down to "Health Issues".