After Dumbledore died during the Battle of the Astronomy Tower in 1997, Fawkes sang his Lament to the assembled mourners. According to a Reddit post, Fawkes left Hogwarts to live life alone and was never seen again after Dumbledores death. It is not clear what happened to Fawkes after that, but he did not appear at the top of the Astronomy Tower when Dumbledore was killed by Severus Snape.