I have been sue for non-payment of a written contract debt

As part of the affirmative defense.  You may also be able to file a pre-answer motion to dismiss based on the statute of limitations.  Remember, the breach of contract statute of limitations runs from the date of breach, not the date of contract.  For example, if the Fl statute of limitations on breach of contract is 4 years, and you took out a loan 7 years ago  but didn't miss any payments until 3 yeatrs ago, the claim is timely.