If you believe the reason for the ban happened when the account was outside your control you need to call Support, find your local number here
support.xbox.com/.../call-us.aspx, and open an Unauthorised Access case with them.
To enquire further about a ban on the forum you need to post here http://forums.xbox.com/xbox_forums/xbox_support/f/41.aspx providing the name of the banned gamertag, but they are likely to say the same thing as above.
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