In a nutshell, Harry said to hell with it and decided to ditch the whole wizarding world after Trelawny joined the Death Eaters. So he faked his death, and Ron's, and went and joined a Muggle basketball team. But that wasn't a good enough disguise so he underwent a magical sex change. But Cho found him and persuaded him to come back. Only that meant the bad guys found him as well. Thankfully he was able to escape their clutches with the help of a couple of the robots from Transformers. Then he was reunited with the Weasleys and went back to Hogwarts, where he was given a teaching position under McGonagle as Headmistress, and Snape was demoted to Filch's position.
Yeah. Too much coffee, I think.