I had a fundoplication about 15+ years ago, and recently started having really bad heartburn/reflux.. is this a sign of the fundo failing?

Maybe...: 15 years without reflux after a fundiplication is an excellent outcome. It's possible the wrap has slipped or you've developed a hiatal hernia. Make sure you are avoiding dietary reflux triggers and try taking an acid blocker (e.g. Zantac (ranitidine) or Prilosec daily) and make an appointment with a gastroenterologist who will likely want to offer you an endoscopy to see whats wrong. Good luck.

Yes: Recurrence of reflux symptoms after fundoplication is usually a sign of it failing. You should consult with an experienced anti-reflux surgeon who can thoroughly evaluate you and re-do the fundoplication if necessary or convert to a different type of anti-reflux surgery. This is safe when done by surgeons experienced in revision anti-reflux surgery.