Can a mandatory trigger be missed?
You should pretty much always call a judge when something weird like that happens. Don't think of this as starting a confrontation with the judge or with your opponent - judges are simply the experts in how to fix weird things, and in doing so such that both players will usually think they were treated fairly. That said, if you called the judge over, they would probably just put the missed trigger on the stack at that point. Here's the guidelines from the Judging at Regular Rules Enforcement Level (REL) document (emphasis added):


  [Triggered] abilities are considered missed if the player did not acknowledge the ability in any way at the point
  that it required choices or had a visible in-game effect. If the ability includes the word “may,” assume the
  player chose not to perform it. Otherwise, put the ability on the stack unless you think it would be too
  disruptive - don’t add it to the stack if significant decisions have been made based on the effect not
  happening! Unlike other illegal actions (which must be pointed out), players may choose whether or not to
  point out their opponent's missed triggers.


Since the trigger was mandatory and since no significant decisions had happened since, the bold line says that the ability should go on the stack when noticed.

(At higher RELs, the Magic Infraction Procedure Guide has a lot more to say on how to handle missed triggers. You can read that yourself to see how your specific case would be handled in a Competitive setting, but ultimately calling over a judge and letting them handle it is the right move either way. By Tournament Rules, intentionally missing your mandatory trigger is Cheating at any REL.)