A lawsuit was filed against me for a monthly bill that I fell behind on. I have since paid the bill and I'm current. How do respond to the lawsuit in writing?

Has a court hearing been scheduled? I would write to the creditor/plaintiff and tell them that, since, you have brought the account up to date, you are expecting them to withdraw the suit and that you will expect confirmation of that withdrawal by return mail. Send a copy to the court where the suit was filed, with the docket #.

Keep tabs on this, if a hearing is scheduled, you want to make sure you appear with proof the account is up to date.