Firstly I would put her in a cardboard box and bring her in the house where it is warm. Once she has acclimatised to the warmth (a few hours) I would give her feet a soak in warm Epsom salts and examine them thoroughly for any signs of scabs that would indicate bumblefoot. I would also offer her warm water with a vitamin supplement and warm moist food like scrambled eggs or a mash made with her regular food soaked in warm water. If no sign of bumblefoot scabs/swelling, then examine her legs for any injury or swelling. How old is not too old? Has she ever been lame like this before and recovered? I'm thinking this might possibly be Marek's disease or Lymphoid Leucosis. If she is a silkie, it may also be a peck to the head that has caused damage to the brain resulting loss of control of limbs, or as others have said, a simple injury. Either way, she will need to be kept warm and have safe access to food and water. Be aware that if she is left somewhere that rats can get access, they could eat her alive, so please make sure she is somewhere safe. It might be helpful to make her a chicken hammock or sling to support her in an upright position if she is struggling to stand. This would be particularly useful if she has a break of strain as it will support her whilst it heals. It can be made from an old Tshirt placed over a suitably sized cardboard box with the open end upwards and holes cut in the fabric for her legs to fit through, so that she is suspended in the box. It may be necessary to gather and staple the fabric to the sides of the box to get the tension right so that she is supported off the bottom. Then cut a hole ender her vent so that poop can drop through and place a tray with some litter below to catch it. Yoghurt pots containing food and water can also be inserted into slots in the fabric in front of her.
Edited to say, disregard possible Silkie brain damage comment as photo now posted indicates that she isn't one
Edited by rebrascora - 1/3/17 at 7:12am