"But what happened to Vol--, sorry -- I mean, You-Know-Who?"
"Good question, Harry. [Hagrid] Disappeared. Vanished. Same night he tried ter kill you. Makes yeh even more famous. That's the biggest myst'ry, see... he was gettin' more an' more powerful -- why'd he go? Some say he died. Codswallop, in my opinion. Dunno if he had enough human left in him to die. Some say he's still out there, bidin' his time, like, but I don' believe it. People who was on his side came back ter ours. Some of 'em came outta kinda trances. Don~ reckon they could've done if he was comin' back. Most of us reckon he's still out there somewhere but lost his powers. Too weak to carry on. 'Cause somethin' about you finished him, Harry. There was somethin' goin' on that night he hadn't counted on -- I dunno what it was, no one does -- but somethin' about you stumped him, all right."

Harry Potter and the Philosopher's Stone - Chapter 4 : The Keeper of the Keys
See this quote in the Pensive -