The best advice I could provide would be to tell you to get a lawyer asap. The problem could be as simple as you having forgotten that you applied for the card to as complicated as someone having forged your signature and stolen your identity.
DISCLAIMER: This answer given is for general information purposes only. No attorney-client relationship exists or is intended. If you have additional questions concerning this matter you may feel free to contact me at the Law Offices of Jacob J. Linhart, P.A., (561) 632-6982 or Jacob@LinhartLaw.com.
I strongly recommend you contact a consumer attorney right away. If the alleged debt was never yours, you will have an excellent defense, but it needs to be asserted in the proper way or you will lose the ability to defend the case. Depending on the circumstances, you may also have a claim against the debt collector and/or its lawyer for bringing the lawsuit against you and forcing you to defend against it, since they falsely representated the amount, legal status and character of the debt when they sued you. Please see the links below for more information about your rights.