Seems like a sinus/respiratory infection to me. I would recommend purchasing some Tylan50 injectable from a livestock supply store and beginning treatment. The Tylan50 injectable (there is an oral form, but it doesn't work as fast) dosage is 1cc for large-fowl, .5ccs for bantams, injected into one side of the breast once daily for 5 days. Alternate the side of the breast that you inject into, and use a small gauge needle, as Tylan can make the injection area sore. Do not give Apple Cider Vinegar, probiotics, or dairy products during treatment. Improvement should be seen after 2-3 days.
You could also try another antibiotic, like a form of Oxytetracycline (Tetroxy HCA-280, Terramycin, Duramycin, etc.), but that antibiotic is weaker and less effective against respiratory diseases.
Besides treating with an antibiotic, I would isolate this bird, as respiratory infections are often contagious. Keep her in a warm, clean place, and minimize stressing her out. Perhaps put some poultry vitamins/electrolytes in her drinking water to boost her strength. If she doesn't eat, encourage her to do so by feeding treats like scrambled eggs, moistened feed, applesauce, etc.