There were four reasons why Constantine was not filming for the second season. First, the lead actor, Keanu Reeves, had a conflict with another movie, The Everlasting, which made it impossible to continue filming for the second season. Second, the original investor, Warner Pictures, was not satisfied with the box office of the first movie, so the producers did not want to continue investing. Third, the director was hesitant, which might be one of the reasons why the second season was stopped. Finally, no one was willing to continue investing in the series. Although there were reports that Constantine was expected to be resurrected for the second season, there was no definite news at the moment.
The Sandman isn't real in the sense of being a living, breathing entity. In the Sandman real story, it's a fictional concept. It's a story made up to explain sleep and dreams, especially for children. Just like many other mythical creatures, it exists in the realm of imagination and stories, not in the real, tangible world. We can enjoy the stories about the Sandman, but we can't expect to meet him in real life.
Well, it all started with a strange accident involving some sort of magical dust or substance that transformed him. But the details can get pretty complex.