Hey Arnold! The Jungle Movie was supposed to be the ending this series needed. Following up on the finale, The Journal, it would have chronicled Arnold's search for his parents and gave fans closure.
I've been rewatching this show recently during a nostalgia stint (Hey Arnold, Doug, Pete and Pete, Digimon Adventure), and it still upsets me to this day that we never got a proper ending to this show. Hey Arnold The Movie wasn't even supposed to be a movie, it was supposed to be a three part tv special, while this was supposed to be the only theatrical release. Plans changed to make both theatrical releases though, and after the first movie Craig Bartlett got tied up in contractual issues and The Jungle Movie inevitably got cancelled.
Over the years there have been several rumors of the movie coming back, but with each passing year its release becomes less and less likely. The fanbase is alive though: Hey Arnold's Second Film: Fans Still Fight for It
Craig Bartlett still seems interested in doing the movie, and there is a pretty big fan community that continues to grow that wants to see the movie. I've joined this community, signing the always (seemingly) hopeless petition, joining the facebook page, and writing letters, all methods mentioned in the above article.
Things get cancelled all the time. People are still butthurt about Firefly, but inevitably it got a movie ending. I'm a fan who showed great disappointment in the cancellation of HBO's Bored to Death, but there is supposedly a movie in the works for that series with the intent of wrapping it up. I don't know why, but getting The Jungle Movie made just doesn't feel completely hopeless to me, at least not yet. So if you are interested in seeing it made, I implore you, join the fan community and take the simple steps available to us to show our interest.
If you want to write a letter and didn't read all the way through the article above, here is the nifty image provided to get you on your way: