The Son of Neptune - Heroes of Olympus - By Rick Riordan

Percy Jackson, son of Poseidon, God of the Sea, has woken from a very deep sleep and come face to face with two snake-haired ladies who refuse to die. But they're the least of his problems. Because Percy finds himself at a camp for half-bloods, which doesn't ring any bells for him. There's just one name he remembers from his past.