What should players do if a trigger was last valid over a turn cycle ago?

When a triggered ability is missed and it has been last valid over a turn cycle ago, the correct action is to instruct players to continue playing. This is in accordance with the rules surrounding missed triggers in Magic: The Gathering, specifically within the context of the Infraction Procedure Guide.

Missed triggers can only be addressed if they occur during the current game turn. If a trigger was last valid over a turn cycle ago, there is no way to resolve it to the state of the game as it was previously, and thus it is considered too late to take any action regarding that trigger. Instead, players should continue to play normally, as the game has progressed beyond the point where the trigger could have had an effect.

The other options—restarting the game, issuing a penalty, or notifying a judge—are not applicable in this situation. Restarting the game is not warranted for a missed trigger, as it would be an excessive response. Issuing a penalty would only apply if the missed trigger had not been addressed under the applicable guidelines and was still valid. Notifying a judge may be appropriate under certain circumstances, but if the missed trigger is simply too old to act on, players are simply encouraged to continue playing without any further intervention required.