Abandoned can use ideas

On his 18th birthday a year after the Seven defeated Gaia and saved the world, tragedy struck. The Lost Hero, Percy Jackson, was killed in a attack on their home Camp Half-Blood. For the past 5 years, his soul has been in the Underworld being honored for his achievements. But, when what would've been his 23rd birthday and the anniversary of his death, his soul vanishes from the realm of the dead. Has someone returned his soul to the living realm? If he was back, then where was he?