Did Arnold's parents ever come back?
Hereof, how did Arnold's parents die?
"The grandparents are mentally unstable and told Arnold that his real parents, of normal age, died in a plane crash. There's one type of hydrocephalus, the one Arnold suffers from, that's called the Arnold Chiari syndrome. That's the reason why the creators decided to call the boy Arnold."
Correspondingly, did Arnold ever find his parents?
In the movie, Arnold battles pirates to find his parents. It turns out they're ill with sleeping sickness. Arnold, Gerald, and Helga somehow figure out the cure for the illness. They save his parents and the whole village, and he finally brings his parents home to the boarding house in New York.
Original Hey Arnold composer Jim Lang confirmed the studio has no plans to bring the show back, but Netflix has confirmed it's going to make the sixth season of the classic toon. The Jungle Movie but Jim Lang said the film didn't bring in the new viewers they wanted.