What is the old show on Nickelodeon called when a boy dies and only his best friend can see him?

The show you're referring to is called "Danny Phantom." It follows the story of Danny Fenton, a teenage boy who becomes a half-ghost after an accident with a ghost portal. His best friend, Sam, can see and help him navigate his new abilities and the challenges that come with being a ghost. The series blends action, comedy, and supernatural elements.