It's important if you're sued for a debt that you do not owe, to make sure that you don't ignore it because if you do, there is label to be a final judgment against you in the entire amount due, and that would last for 20 years, and subject you to debt collection, like wage garnishment and so forth, so don't ignore it.
As for what you can do, there's plenty of defenses. They should probably be raised by an attorney but you definitely want to dispute the debt, raise the defenses in the court case, send in a copy to opposing counsel. You can ask for an extension of time if you can't get to an attorney quick enough. Usually, when they serve you with the summons, it takes about 20 days for you to have a response due, so you'll want to schedule an appointment within that first 20 days to provide the right answer.