This page explains how to unlock the New Game + feature in Chrono Trigger, which allows you to see new endings to the game.
Unlock New Game +
To unlock New Game +, you must first complete the game by going through the Black Omen dungeon, which leads to the final boss. After you defeat the final boss and watch the ending, reset the console. This will unlock a new option on the Load Game screen: "New Game +". If you select it, you can choose a save slot to create a new game with. The new game will be based on all the stats and items from the save slot that you selected. To make the best use of this, create a saved game just before fighting the final boss, then use that saved game to create a New Game +. You can save your New Game + to a different save slot if you wish.
In the New Game +, be sure to equip Crono with the best equipment, because he will start out with his original equipment at first.
The New Game + feature allows you to reach all the possible ending scenes in the game. The endings depend on when and where you beat the final boss. See the Endings page for more details. There are also notes throughout this walkthrough that explain when in the game you can get each ending. In some cases, the only way to reach certain endings is through a New Game +, because you either don't have access to the final boss at that point in a normal game, or because you would normally be too weak at that point in the game to defeat the final boss.