You might be able to re-enable it via this page : http://appleid.apple.com, then 'reset your password'
If you have an iPhone, iPad and/or iPod Touch you might then need to log out of your account on it by tapping on your id in Settings > Store (Settings > iTunes & App Stores on iOS 6) and then log back in so as to 'refresh' the account on it
If that doesn't fix it (and if iTunes Support disabled the account then it might not) then you might need to contact iTunes Support : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page