You didn't know this, Sarah?
Originally, Dr. Donald Blake was similar to other Marvel protagonists. He was an ordinary man, a surgeon, who had to walk with a cane due to an accident. While vacationing in Norway, he happened upon a cave that had once been used by the Vikings in ancient Norse times. There he found Mjolnir, the hammer of Thor, with the inscription,
"Whosoever holds this hammer, if he be worthy, shall possess the power of Thor." Dr. Blake picked up the hammer and was instantly transformed into Thor, the God of Thunder. He also had the ability to transform back and forth from Thor to Donald Blake, and when he did, the hammer Mjolnir took on the appearance of his walking stick. However, later on it was revealed that Dr. Donald Blake did not exist, and his entire persona was the creation of Odin, and that Blake had actually been Thor his entire life and never knew it. When Odin banished Thor from Asgard, he decided to take away his power, immortality, and his memories, and had him be reborn on Earth(Midgard) as a mortal man, living a mortal man's life, in order to teach Thor about humility, compassion, wisdom, temperance, and humanity, so that he would one day be worthy of wielding Mjolnir again.
When Thor finally regained his memories and realized who he actually was, he retained the memory of his entire life as Dr. Donald Blake, from childhood to adulthood, including being a surgeon, but the Blake persona was gone.