Constantine, if you didn't know, is a TV show that aired between 2014 and 2015 about the DC Comics character, John Constantine from the comic series Hellblazer. Unfortunately, it has been cancelled after only one season. However it does have a crossover episode with Arrow season 4 episode 5. Meaning that Constantine is now a part of the DC TV universe.
Constantine was a great show, extremely mystical and showed the darker, more mature side of comics. It introduced many mystical DC characters and created a very compelling story and the fans of the show want it to continue.
Producer of Constantine, David S Goyer stated that he doesn't think that the show should have been on NBC:
“We almost doubled our numbers in DVR numbers, but they weren’t quite there in network television in counting those metrics,” he said. “If it had been on a basic cable channel, it could still be on.” - Variety
Which is disappointing, but there were rumours that Syfy were going to buy the rights and continue the series under Hellblazer (like the comic) but nothing has come from it yet. We also can't rely on the CW taking it on because they already have 4 comic book adaptations on their channel as it is with Arrow, The Flash, iZombie and the upcoming Legends of Tomorrow.
Which leaves the show, ironically, in limbo. Personally, I'm hoping the crossover with Arrow is the boost it needs to live again.