How did I get hacked?

I logged in today after not playing for like, a year, to give a friend an item, and all my stuff is gone. I don't really care about the stuff, because I don't plan on playing MapleStory anymore, but I'm curious as to how my account security was compromised without me so much as visiting a MapleStory-related website besides this forum. I changed my password regularly before I quit, it wasn't something easy to guess, and no one had my account info. Can anyone offer an explanation?