Whatto do if I am being sued by a debt collection agency for a debt for which I do not know who the original creditor was?

Contact the court; ask the judge's clerk how you would get a continuence or adjourment to later (a delay), explaining that you have not been provided with any indication or proof of the original creditor and the debt. You may need to go to court and ask the judge on the day of the trial--do NOT miss the trial date. If the judge is not prepared to grant the extra time,  come prepared to question the other side on the spot--they'll have to answer then. The issue about whether you can get extra time or not will probably turn on whether you had any chance to either use the mechanisms of "discovery" (e.g. requesting documents; requring answers to written questions) to find out this information earlier, or at least had the opportunity to request a delay earlier. If the court concludes that you "slept on your rights" and did not take action when you could have, it may be disinclined to give you a delay. In that case, as noted, do the best you can in trial and then be prepared to appeal if after the fact you can prove that this is not a valid debt or that it has already been satisfied. (If it is a valid debt, you'll almost certainly have to pay it.)

If it's a large enough debt, you may need to later also consider bankrutpcy as an option.