My Apple ID was disabled how to activate it again and use it back

Depends upon why it's been disabled. If for security reasons then resetting the password might fix it : If your Apple ID has been locked - Apple Support

You might then need to log out of your account on your phone by tapping on your id in Settings > iTunes & App Store and then log back in so as to 'refresh' the account on it.

If that doesn't fix it then contact iTunes Support : http://www.apple.com/support/itunes/contact/