In 2002, Hey Arnold!'s initial series finale "The Journal" aired. This one hour special showed Arnold finding his father Miles' journal which was written during his time spent in San Lorenzo. Here we witnessed him meeting Arnold's mother Stella, them getting married and having Arnold, and battling a disease that's been plaguing a jungle in Central America.
The episode was left on a cliff hanger as Arnold finds a map routing his parents previous trip in San Lornezo. This would be the setup for Hey Arnold!: The Jungle Movie. It was originally a theatrical feature, but Nick executives opted to theatrically release the TV made movie now known as Hey Arnold!: The Movie instead. However, in 2004 (according to Hey Arnold!'s wiki page-http://heyarnold.wikia.com/wiki/The_Jungle_Movie ) when series creator Craig Bartlett declined to sign an exclusive contract with the studio, which would mean his creative direction would be limited and he'd only be able to produce projects for them, Nickelodeon effectively terminated TJM and the series.
Yet, even after a decade fans and Bartlett are still working to get TJM made. In 2012, Bartlett said he'd,"have to start from scratch." Then a year later, Bartlett announced he's working on a new Nick show Sky Rat and wants to focus on the new project before continuing with TJM. And in 2014, a rumor came out that TJM was back in production and slated for a theatrical release in June 2015 or 2016, but this hasn't been confirmed and is most likely untrue.
Yet, strong fan response has been getting noticed as Nickelodeon president Cyma Zarghami mentioned to Bartlett that they were receiving tons of letters about Hey Arnold! in 2013. Advocates for TJM have been composing letters
and holding a petition, which you can sign here-http://www.thepetitionsite.com/313/623/108/opruth-green-light-hey-arnold-the-jungle-movie/
And you can join a group on FB, as long as you follow their rules, which supports and gives updates about TJM. As of now, it has over 24,000 members.