for a really really long time like 2-3 years?

Don't think so, I think I went on about a 3 months ago and my chars was still there and i hadn't played in years

pretty sure they don't reset chars or delete accounts.
Take Fangblade one of the top ranked character, that guy hasn't played for years and the fact he's still on the leader board means his account is still there just inactive.

only other thing i can think of is a hacker making it seem like your account was reset.

also there's a Maplestory subforum here http://forums.d2jsp.org/forum.php?f=261

Well... one of my account that was hacked back in 2005 was deleted. after Nexon takes over, i don't think they will remove or reset any accounts.