First, double-check that the debt was included in your bankruptcy. If it wasn’t, you might still have time to include it.
Do not disregard the case; else, a default judgment will be entered against you. Respond to the lawsuit by filing an answer in the court where you were sued, claiming that the debt was discharged through bankruptcy. When the judge discovers that the debt was listed in bankruptcy and then discharged, the case is usually dismissed. If the judge does not dismiss the case, you can seek an injunction from the bankruptcy court ordering the creditor to stop pursuing you.
A bankruptcy discharge is a valid legal defense against any debt that has been duly discharged, but you must assert it.