The answer is nothing. They remain in exile. Nothing is telling you to do anything to the exiled cards when Ashiok leaves the battlefield, so they’ll remain in exile. Even if Ashiok returns to the battlefield, the new Ashiok is considered an entirely different permanent that has no link to the previous life that Ashiok had, so the new Ashiok will not have access to the cards that the old Ashiok exiled. Those cards are stuck in exile forever, doomed to live an isolated existence away from the rest of the cards for the rest of the game.
Today’s Rules Tip written by Nathan Long