Lately a number of people have reported problems with being prompted for their CD key again. If this happens to you all you need to do is re-enter your CD key, make 100% sure that you entered it correctly, and it will work fine.
If you continue to have problems, and you are certain that you entered the key correctly and that no-one else has used it, please get in contact with EA support who may be able to assist you further.
EA Support:
http://support.ea.com/