Have you looked at http://www.texaslawhelp.org?
I did the world's quickest look at TX civil procedure and I can't find anything that has "reactivated" in the title. As you and your ex both got notices, you and your ex both must appear. It isn't the clerk of the Court you want to talk to, it is the judge's assistant or the bailiff who works for the judge.
You can write that person a letter (before May) and explain what you said here - that there is no outstanding child support, that all issues were addressed eighteen years ago, the divorce is final, and you are at a loss to understand why the court wants you to appear. The court may have some perfectly good reason why you should go deal with something, this could be a computer generated screw-up with a similar cause number, or perhaps there was a document that was supposed to be filed but was overlooked at the time.
Don't ask to talk to the judge, that is not proper, but you can speak with his or her assistant. Alternatively, you can go to the Courthouse, and look in your file and see if you can figure out what triggered this notice.
Your county Bar association may have a clinic or a class where you can talk to a family law attorney, or maybe you want to set up a short consult with a local attorney to get this straightened out. Take the document you received from the Court with you so the lawyer can read it themselves. Hope this helps. XXXXX XXXXX
Hope this helps. XXXXX XXXXX