The movie was about a young boy, who started as a real boy but went inside his own dream and was given the keys to the city, but he accidentally unlocked a monster and then returned to real world, but had to come back to the fantasy world to undo what he did. What is this movie called.

Sounds like Little Nemo: Adventures in Slumberland (http://en.wikipedia.org/wiki/Little_Nemo:_Adventures_in_Slumberland)