She stopped laying a few months ago. Suddenly she can't stand or walk.
Putting your location in your profile is helpful - you mention that you left her in a shady spot and she hasn't moved since yesterday - can we assume you are not the US?
Photos are always welcome
Is she molting?
With the symptoms you describe (stopped laying, can't walk/stand) - check her over well for any swelling/bloat or feeling of fluid in the abdomen. It could be that she is suffering from an internal laying/reproductive disorder like Egg Yolk Peritonitis, Ascites, cancer, tumors or Salpingitis.
Having a fecal float to rule out worms is always a good idea, check her crop to ensure it's emptying overnight, look through her feathers for lice/mites and feel the legs/joints just to make sure that injury is not the cause of her sudden difficulty.
It's possible that she may also be presenting with a disease like Marek's, there is not cure for this, so you would want to exhaust all other avenues. Encourage her to drink and eat - add some poultry vitamins to her water and entice her with some chopped egg.
If she is dirty, then giving her a clean-up is a good idea - I do suspect you are in a warmer climate, but it's always best to make sure she doesn't chill as suggested by
@ChickNanny13 She needs to be completely dry before going back outside. You can give her a soaking in a warm bath with a little dishsoap or baby shampoo - work the soapy water through the soiled feathers to loosen the poop. For hard clumps, a pair of pliers works well to break that up, if the clump is too difficult to break loose, then you can trim it out with scissors - try not to pull the skin, it will tear easily. Wear rubber gloves and if you happen to want to trim any dry fluff (feathers) around the bum, then I advise that you wear a mask - the fluff is very fine and you don't want to inhale that.
