Perhaps your login to minecraft failed, and you clicked play offline. If you're on offline it uses the playername player, which can't be used because it has been banned. Or it can be that you used the playername player before, and that account might have gotten IP-banned instead of just banned. In any case, I think this happened by accident. Usually you receive a warning before getting banned. I will unban you now, but you might get banned again by Koolio for reasons unknown to me.