Have you tried restarting the server? The only two things I can think of are a plugin not working right, and something went wrong when the server started up. And no, without hacking your computer, no one could've hacked that. In which case you probably have bigger problems to worry about than someone in creative. But please don't overreact to this, the chances of that happening are less than a million to one.
EDIT: Hm. Someone beat me to it, guess there is some weird hack. I'd call it a bug, but meh.