After defeating Mrs. Shoggoth and getting back to the living world (which was probably just half an hour in Living world time), Lydia decides to introduce her new family to her dad, Maitlands and friends. As they're all mingling and Beej is back to his old self, Lydia collapses unresponsive as she falls into a comatose. 

But Beetlejuice knows better. Somehow, someway, her soul was taken and is now trapped in the Netherworld. 

Found this quiz and couldn't resist. I had to share.

https://www.women.com/alex/quiz-can-you-ace-the-ultimate-beetlejuice-quiz