Well, I am not staff. But, there are a variety of different reasons you could be banned. Is your Gold Membership just expired? Not banned, just need to renew it? Do you have your xbox JTAG'ed? Any sort of hacking that is detected will ban your console and flag your GamerTag. If one of your friends has their console JTAG'ed and you logged into Live on it without knowing it was hacked would get you banned. Excessive swearing, racial slurs, spamming people could do it.
You're not giving us a whole lot to go on, so I can't be 100% sure. Did you buy the xbox from ebay? Possibly the person before you hacked it. Again, just too many unknown variables.