We are fellow users, so obviously nobody on here will know why a particular account is disabled, but the most common reason is likely to be too many incorrect password attempts
Try resetting its password (and then logging out and back in everywhere) : If your Apple ID has been locked - Apple Support
If that doesn't fix it contact iTunes Support in the country where you are (and which is therefore the country on the account) and see if they can/will re-enable it : http://www.apple.com/support/itunes/ww