If you can show that the summons and complaint were never served on you, or at least were served in some improper fashion, such that you did not have notice, you may be able to have the judgment set aside. In that case, you would have to file and answer and the case would resume at that point. You would do this by motion. Ideally, retain an attorney to represent you; otherwise, you will need to look to your state's court rules for the proper procedure. You can use you own testimony as evidence; the other party will then have a chance to present its evidence that service was made.