You should ideally thaw out chicken before cooking it. The reason being that the centre of the chicken cannot be reliably cooked to the necessary temperature, as you have just found out, and they can go tough.
If you have one, I would put it in the micowave for a blasting. Otherwise you might like to try boiling them. Depends on what you are tying to do. It will probably take quite a long time to cook them in the oven though.