I'm not really sure what happens to them, however, I do go through them periodically and see if I'm able to answer any, which kicks them back alive again. Someone sees my answer and then would have the opportunity to chime in with their own answer.
I once found a link on here to questions that had zero answers. Not sure how to get back to that page. If anyone knows, please leave me the link. Thanks!
KUDOS@ Rooster
If you go to Topics and choose Unanswered,you'll find long lists of them. As far as I know, you can go past ten pages of them and still find them.