What happens when you close your eyes too long while wearing contacts?

You can't see! There really is no danger keeping the eyes closed for long periods while wearing contacts, unless you sleep with contacts. People with dry eyes can feel as though the lenses are stuck to the eyes, but with proper care and lubrication, there should be no concern.
Usually not much. If you are referring to closing your eyes periodically throughout the day while still awake, nothing will happen for the most part. However, if you are referring to sleeping, then it can be a dangerous thing to sleep in your contacts because it increases your chances of infection.