Sometimes it's just the key (moreso the GUID). Just switch it out and change your name for obvious reason. Sometimes it's an IP ban along with your GUID. Just change out your IP and key. And sometimes it's an HWID ban n which case...well uhh...reformat...but even then, sometimes that doesn't work
Also, several threads have been about the same thing (How to get around a ban). Next time I suggest doing a bit of digging or else people like myself will bitch at you.