My name is XXXXX XXXXX I'd be happy to answer your questions today. I'm sorry to hear that this happened.
First, you must respond to the lawsuit. If you don't, the judge will enter a judgment against you, and you will have forfeited the right to defend yourself. That's important. Second, the bank has the burden of proving that you owe the debt. If you paid all amounts that you owed, that is a valid defense to the action. If you reported the card stolen, and charges were made after that, that's another defense. Banks keep statements for quite awhile. If you ever had access to Wells Fargo's website, you may still be able to log in and download statements from the relevant time period. If not, after you respond to the suit, you have a right to send them a Request for Production of Documents, demanding that they provide you with copies of all relevant charges and statements.
To file a response, the defendant goes through the complaint and admits or denies each allegation, or states that he lacks sufficient information to admit or deny the statement. Do not admit anything you're not sure of - if you don't know your old account number, and you don't have the statements, don't admit that the account number is XXXXX (I've actually seen companies put the wrong card number in the complaint). State that you don't know, and ask that they prove it. Then, at the bottom, you assert your defenses. Here are some examples that show what an Answer might look like (you would use the court caption as shown on the Complaint and Summons).
An Answer should also contain a certificate of service, usually printed at the bottom of the complaint after your signature (or you can attach a separate page). It usually says something like:
I, NAME, hereby certify that on, DATE, I served the above Answer on the plaintiff by mailing a copy, via first-class mail, postage prepaid, to ADDRESS.