What happens if someone slept before they removed their contact lens?

Usually nothing. But some kinds cause you to get dry eye, eventually you could get an eye infection.

well I have done it a couple of times. Once on a train journey, I felt too sleepy for the train to stop at a station and since it was jerking and there was the fear of the lens falling somewhere while opening it, I slept right on with it inside my eyes. Nothing much happened, only I had red swollen eyes but they were OK after a vigorous splashing of water and gradually became normal. Another time was at a friend’s house. I had gone in the morning and planned to return before lunch. But one thing let to another and her mother insisted on me staying back for lunch and after that we were just lazying on the bed and I felt soooo sleepy (typically me). I did not want to worry her about searching for something to keep my lenses so I just went to sleep with them inside. Same swollen eyes, same splashing of water and OK after a few hours. But its not a very good practice.

Some contact lens can be slept in. If it is a daily wear only lens then your eye might swell up, get dry, develop and ulcer or corneal abrasion....which can be very serious.

Depends on the contacts. I have a pair that can be worn for over 24 hrs straight including sleeping, however whenever I do it my eyes are very dry and the contacts need to be taken out rinsed in solution then placed back in. If your curious about your lenses I would contact your eye doctor.