How do I turn my Apple ID back on

If by 'turn on' you mean that the account has been locked or 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 and see if they can/will re-enable it : http://www.apple.com/support/itunes/ww or https://www.apple.com/emea/support/itunes/contact.html