They have the burden of proving that the account exists and that there is a balance due. You can avail yourself of discovery under the rules of the court in which the action is pending - the availability of discovery can vary by court - and try to get copies of original documents via a request for production, and other information through interrogatories or requests for admission. (If available you can consider depositions, but they're more costly and much more difficult for pro se litigants to conduct.) Your state's court rules are here; also check with the court about any local rules.
You need to make sure you file a timely answer and affirmative defenses.
Creditors are not required to report debts to credit reporting agencies, so the absence of a debt from a credit report has no legal significance.

