Terminan Apocalypse

The is the ultimate fate that approaches over the land of Termina. 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. This is the 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 the Clock Tower at the center of Clock Town before crushing the rest of the entire town under its weight. As a direct consequence of this impact and the fact it occurred in the very center of Termina, a powerful, fiery, dense shock wave is sent throughout the entire land, destroying everything in its path and wiping out all inhabitants, including Link, who, no matter where he is in Termina, or in any form, even as Fierce Deity Link, presumably dies from the explosion.

Majora's Mask is then seen receding into the fiery darkness and fading to black, similar to the Happy Mask Salesman's narrative at the start of the game. But, somehow, the Salesman asks Link what happened during this moment, 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 will resume his quest since the last time the player played the Song of Time. In, if Link wishes continue from the previous three-day cycle, the player must reset the game in order to resume from the last time it was saved.

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 still try to stop the Moon at all costs, only to fail to do so. Link will then have only one minute to play the "Song of Time" and go back to the First Day. If the minute lapses, the Giants will collapse to the ground due to the unrelenting force of the Moon, and the apocalypse will continue as if the giants were never called, inevitably leading to Termina's destruction.
 * This is one of the few instances of a Game Over in the Zelda series that does not involve Link dying after fully depleting his life energy. Some of the other instances include: if Zelda is killed by Vaati after his ceremony (following the third chime of the bell) and he claims the Light Force in, when Telma's wagon is totally burned in while she and Link try to escort Ralis to Kakariko Village, when a passenger of the Spirit Train is captured without the Pirate Hideout being unlocked in  (which is actually the case when Carben is traveling with Link to the Ocean Sanctuary), and if The Imprisoned manages to reach the Sealed Temple in SS undefined.
 * The Moon crashing down into Termina is the Bad Ending of Majora's Mask.