You fell into Mt. Ebott, or rather you were thrown in. Toriel found you wrapped in a little blue blanket and carried you out of the ruins to the brothers Sans and Papyrus. She explained the situation, and although the brothers were puzzled by the blanket, they decided to keep it. While Sans was napping, he heard a loud cry under the blanket. He approached it, removed the cover, and found you, a crying baby. It seems that you were hungry.