What happens when a creditor is not listed?

It depends on which bankruptcy chapter you filed and whether any assets were distributed.  You didn't provide any of that basic information, so it's very difficult to answer your question. If this was a Chapter 7 case and no assets were disbursed by the Trustee, then you were discharged from the debt even though the creditor was not listed.  Of course, if the creditor has a basis to object to the discharge of your debt (e.g. fraud), then they can still seek to reopen your bankruptcy case to litigate that, but otherwise the case law states that the debt was discharged and they cannot take any collection actions against you.