When Happy Day first appeared in 1974, The Cunninghams had 3 children. Richie, Joanie and Chuck. Chuck was the oldest child. He was not seen very often in that first season. In the second season of the show, it was said that Chuck was off to college, though I don't think he ever appeared before the camera.
By the third season, there was never any mention of Chuck anymore and seasons thereafter. He just disappeared without any mention of what happened to him.
Was he abducted by aliens who brainwashed the minds of all the characters to make them forget of his existence. Is he hanging out with Jim Morrison and Elvis. Did he die of a drug over dose. No. What happened to Chuck was the Fonz.
Henry Winklers character became such an overwhelming aspect of the series that there was no need for Richie to have a big brother anymore. Heck after, Ron Howard left the series, the Fonz took over as he had been the star of the show for the many previous years/seasons.