I'm sorry to say but my guess is she has the neurological form of Marek's. Prognosis isn't great but some have treated their birds, although, usually they aren't quite the same and can continue to shed the virus to others. I've heard of someone treating with Hypercium. You can get the homeopathic at Super Supplements or a health food store (are we still calling them that these days?). Hypericum is suppose to treat neurological problems.
Our vet had us try an anti-inflammatory because generally there is inflammation around the sciatic nerve which is why they can't walk/stand. Unfortunately, the bird we were treating developed respiratory issues from the virus and we had her put to sleep. Sadly, being a herpes virus, as our vet said, you can control the symptoms but it could come back, usually if the bird gets stressed. I'm so sorry and I hope I'm wrong, looking at her and seeing the symptoms, it was my first thought.