how did the characters in nightmare before christmas died

In the Nightmare Before Christmas universe, the characters are actually immortal. They were created by Jack Skellington, and he has the power to bring them back to life if they ever die. So technically, the characters in the Nightmare Before Christmas universe can't die. However, in the story, the residents of Halloweentown were originally humans who lived in the real world. When Jack Skellington opened the door to Christmas Town, they were transformed into the monsters that we know today.