Check the bottom of her footpads for bumblefoot, a dark in color round scab. Bumblefoot could possibly be the problem and would require minor surgery.
I have Barred Rocks (love em')...they are heavy birds and it's most likely she jumped down from a high place, like a roost, and sprained or pulled a ligament or tendon. I recommend that you lower roosts and nests if possible and eliminate other high places where they can jump down from and hurt themselves. Place her in a crate or cage with feed and water. She will need rest and relaxation while her injury heals, not walking or scratching around aggravating the injury and making it worse. You can purchase vitamin B complex tablets at a pharmacy, crush a few of them into powder and sprinkle it on her feed. The vitamin B complex may help her injury heal quicker. Continue this for 5 days, then release her and see if she can stand and walk. If not, place her back in the cage and continue treating her for a week. Then release her again and see if she can stand and walk. If not, stop the vitamin B complex in her feed. Sometimes these injuries take about a week to heal, sometimes a month or more, or never. Time usually heals if you have the patience. You can also provide her with scrambled egg mixed with buttermilk in her feed as well. The scrambled egg is extra protein, the buttermilk is a probiotic. I've had this happen to 2 of 3 BR's and 1 Black Star (at different times) and within a week they were back to normal walking just fine. I've had it happen to several roosters. One was able to heal nicely, but the other two I had to eventually cull due to quality of life issues.
