If it is apparently paralyzed legs and/or wings, and not them just not wanting to use them, it sounds like PMV could be the culprit as Biophiliac suggested.
I would remove that bird, and any other birds that appear affected, into quarantine NOW. If you can conclude with some degree of certainty that it is PMV, I would vaccinate all healthy birds (don't vaccinate the sick ones), and treat all of the birds with Viromune or a similar product, which boosts the immune system and can help in treating PMV in infected birds, depending on how bad they are. I would also seek avian veterinary advice as well, if you can.
I have not experienced PMV in my loft, but I'm aware it can be devastating. I have been on the fence as to vaccinating my birds with at least PMV, maybe paratyphoid/salmonella as well.