Under what conditions does this happen? Is it only user-initiated or are there game mechanic conditions that would cause a nation to cease to exist?
1 (Most common): The nation is inactive for 28 days (The user does not log in during that time)
2 (Also rather common): The nation is on Vacation Mode and is inactive for 60 days.
3 (More common that it should be): Moderator Deletion for rules violations.
No nation can be deleted by it's own user, unlike is the case in other MMORPG's.
A nation that dies through through one of the top two methods can be resurrected by filing a Getting Help Request. A Deleted nation can only be brought back if there is a successful appeal... which doesn't happen very often at all.