Before you address the merits, you have to vacate the judgment against you. You have to file a motion to vacate under Maryland Rule 5-535 ("Fraud, mistake, or irregularity"). Your motion must outline legally sufficient circumstances that justify your failure to respond to the lawsuit (e.g. "I was not served, I had no idea the lawsuit was pending").
If a judge vacates the order, it does not dispose of the case. Instead, the case "resets" and you must answer the complaint of your landlord. At that point, you can dispute the debt alleged, and produce evidence that you paid.
I highly recommend you consult with an attorney before you file anything. If you appear in court (and are unsuccessful in vacating the judgment), the creditor will learn your new contact information, and will use it to perfect its judgment against you.
In addition to what my colleague said, then I think the important issue is whether you acted promptly. I've had this issue come up with clients before and frankly, if you had proper notice of the proceedings and simply failed to act, then it's unlikely the default judgment will be vacated. In vacating a default judgment for fraud mistake or irregularity, the "fraud" must be extrinsic -- not intrinsic. The difference is that you must show that you were somehow prevented from defending your rights in the suit at the time -- not relating to what happened during the suit. If you state that you didn't appeal the case in a timely matter, then it's likely you had notice of the suit and didn't do anything about it. A judge will not reward a litigant for failing to take action as required.
DISCLAIMER: Brandy A. Peeples is licensed to practice law in the State of Maryland. This answer is being provided for informational purposes only and the laws of your jurisdiction may differ. This answer based on general legal principles and is not intended for the purpose of providing specific legal advice or opinions. Under no circumstances does this answer constitute the establishment of an attorney-client relationship. For legal advice relating to your specific situation, I strongly urge you to consult with an attorney in your area. NO COMMUNICATIONS WITH ME ARE TO BE CONSTRUED AS ARISING FROM AN ATTORNEY-CLIENT RELATIONSHIP AND NO ATTORNEY-CLIENT RELATIONSHIP WILL BE ESTABLISHED WITH ME UNLESS I HAVE EXPRESSLY AGREED TO UNDERTAKE YOUR REPRESENTATION, WHICH INCLUDES THE EXECUTION OF A WRITTEN AGREEMENT OF RETAINER.