The Sentry is a lot like Spider-man if instead of being bit by a spider he was injected with a serum. He was just a teenager when it happened, so much like Spider-man, the first thing he did was beat the crap out of the school bully. After that he became a superhero and friend to many other superheroes. Everyone loved this guy. He was a mentor to Spider-Man, and a friend to the outcast, The Hulk.
The sad thing is that he had to be forgotten. A dark entity known as The Void came and attacked the Earth, killing millions of people. The Void was the dark version of Sentry, and it was far too powerful to defeat, so he had no choice but to make the entire world forget about him. Once he was forgotten, the Void disappeared. How sad... of course he comes back later, like all heroes do, so it's all good.