You will need to retain a lawyer to review the court's case file and possibly file a motion to quash the writ of garnishment and also move to set aside the judgment. The local Civil Rules, especially Rule 60, has the basis for moving to set aside the judgment. Make sure to assist your lawyer with all of the paperwork and facts that will be needed to tell the story to the court.
If indeed you were not served, your lawyer may have to take the deposition of the process server. If you have a witness or some type of verification that service was impossible, bring that to your lawyer's attention. For example, were you out of town? If so, bring your airline ticket and find someone in the town you visited to verify you were not home. Bear in mind there are ways to start a lawsuit even though you were not personally served, for example, service by publication. Your lawyer can review the court file and tell you what you should do.