Shortly after sending his final letter to his parents, Glimmer of Hope was found dead is his apartment from magically induced heart failure. It was determined to be suicide. The loss of his wife and daughter was too much for him, and he lost hope.
As for why his parents never wrote back or replied to any of his messages. The were killed in a car crash hours after he entered the portal to Equestria.
Try to remember, as long as you keep hope in your heart and in your spirit, life will go on, But if you lose what hope you have, Life wouldn't be worth living. So as long as you have a "Glimmer of Hope" Then you will live forever.