The Sentry, in his time, was a superhero filled with "the power of one million exploding suns," and was arguably the most powerful living being in the Marvel universe. He was also Reed Richard's best friend, a confidant of the Fantastic Four and mentor to the Hulk. Peter Parker even won a Pulitzer Prize for a photograph he took of The Sentry, though neither Peter, nor Richard, nor any other hero from the Marvel continuum (save for the Hulk) remembered any of this.
The reason being that, in order to defeat The Void, his greatest nemesis that also happened to be a manifestation of his subconscious, The Sentry used his immense powers to erase the memory of his existence from nearly everyone on the planet, including everyone at Marvel Comics, and those who had read about his adventures in the past. The Sentry only resurfaced when a virus in his brain caused him to subconsciously implant memories of his forgotten life into comic book writer Paul Jenkins.
Once released from the debilitating grasp of the virus by Emma Frost, The Sentry became a member of, wouldn't you know it, The Avengers.