Terminan Apocalypse

The is the fate that approaches over the land of Termina. In an alternate scenario, if Link fails to play the Song of Time before the 72 hours are up, the Moon continues to fall onto Clock Town, killing everyone who resides there as well as everyone else in the land. Thus, the Terminian Apocalypse represents a Game Over in.

The Events
The event takes place at 6 am on the New Day, right after the culmination of the Final Day. The moon first crashes into Clock Tower at the center of town before crushing the entire town under its weight. As a direct consequence of this impact, a fiery, dense wave is sent throughout the land, destroying everything in its path including Link who, no matter where he is in Termina, presumably dies from the crash.

Majora's Mask is then seen receding into darkness, just like in the Happy Mask Salesman's narrative at the start of the game, before the latter asks Link what happened, just as he did when he and Link first met. Link himself is then shown standing in the inside of the Clock Tower, back on the First Day. Though he and the rest of the land seems to be safe from the tragedy again, all progress the young hero has made during the previous three-day cycle is lost, meaning that he'll resume his quest since the last time he played the Song of Time.

Trivia

 * There is a longer variation of this event. If Link meets the Skull Kid atop the Tower and plays Oath to Order without having freed all of the Four Giants, the freed ones will try to stop the moon at all costs, only to fail to do so. Link and Tatl will then have only one minute to play the Song of Time and go back to the First Day. If the minute passes on, the Giants will fall due to the continuous motion of the moon and Termina is destroyed anyway.
 * This is one of the few instances of Game Over in the series that doesn't involve Link dying after depleting his life energy. Some of the other instances are when a passenger of the Spirit Train is captured without the Pirate Hideout being unlocked first in (which is actually the case when Carben is traveling with Link to the Ocean Sanctuary), when Telma's wagon is totally burned in  while they try to escort Prince Ralis to Kakariko Village, and when The Imprisoned manages to reach the Sealed Grounds in SS undefined.