Why is my Apple ID disabled?

If your Apple ID is locked or disabled - Apple Support

We are fellow users, we won't know why it's disabled.

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