Check for bumblefoot. Look on the bottom of her foot and see if there's a round dark scab on her footpad. If there is....type in 'bumblefoot' in the search box and you'll find a wealth of info about it and how to treat it. If you dont see a scab, she couldve sprained her leg somehow....jumping down from the roost maybe. In that case, I recommend you seperate her and put her in a hospital cage for rest and relaxation (I know, tough to do for a leghorn), but the less she moves around, the better chances it will heal. You can buy vitamin B complex tablets at a pharmacy, crush several of them into powder, then sprinkle it in her feed for a week. Make sure you provide her with fresh water as well while she's in the cage. These things take time to heal, sometimes months. I have a Black Star hen that has been hobbling around on one leg for over a year due to an accident. I have to keep her seperated from the others or they'll pick/peck on her relentlessly. She lays an egg daily and is a real sweetheart. I let her out in a fenced pen to eat grass, take a dirt bath etc...she does just fine on one leg.
