If the summons and complaint were delivered to you and there is proof (in the form of what is known as a "return of service" signed by the sheriff or constable who delivered it to you, then the judgment is valid. However, this doesn't preclude you from attempting to have the judgment set aside. You would need to make a written motion to the court and prepare and sign an affidavit under the pains and penalties of perjury explaining why you did not answer the complaint. In my experience just marking the motion for a hearing often allows the case to be resolved through a payment plan. Of course, you may have defenses to the claim--defenses would include (a) the charges are not yours or (b)the figures on the billings are inaccurate--and those defense should be included in the affidavit. Absent having those defenses in the affidavit, some judges will deny motions of this type. However, as I mentioned above, it's productive just to be able to get in front of the court and speak with the issuing bank's attorney (most of them are quite reasonable to deal with).
If they served you properly then they can get a judgment against you. Even if they did, you can try to fight it in court, and/or pay the judgment and have it removed.