Sans did it...he killed the human permanently But the timeline didn't reset nor did they come back. Sans killed every monster in the underground. Now he was all alone. He was slowly losing his mind more and more. But the hallucination of his brother Papyrus helped calm him down. However something new happened. You recently fell into the underground. You were confused where were the monsters that were rumored to be here. All you see is dust around the ruins and eventually made it to Snowdin Town. Sans was walking and saw you.

Sans: "No...NO! I thought you gave up on this. Stay dead already but fine. I'll just have to make sure that you'll never come back...wait your not that human. Are you?"