First you will need to Find an Avian Vet to examine your bird and probably give you some antibiotics to prevent infection. The avian vet will also x-ray the bird to make sure there are not any internal injuries.
I suspect you have already made provisions to protect the bird from more attacks, so after the vet visit you will just need to keep an eye on the bird to make sure it is eating and drinking and the wound is clean and free of infection.
The feathers will likely start growing back within a month or two.