Board Thread:Questions and Answers/@comment-29430569-20160803190220/@comment-27701762-20160803190854

You likely got what is commonly referred to as the Pacifist Neutral ending. The Neutral route is completed upon defeating Flowey (regardless of whether you spare or kill both Asgore and Flowey).

You should be able to reload your save before the Asgore fight and complete the True Pacifist ending without needing to restart the whole game. Just make sure that you have befriended and hung out with both Papyrus and Undyne, and from there you'll be able to complete the last few pieces.