Do you get any messages ?
If you get a message about its security code then is the address on your iTunes account exactly the same (format and spacing etc) as on your credit card bill : http://support.apple.com/kb/TS1646 ? If it is then you could try what it says at the bottom of that page :
If the issue persists, contact your credit card company and verify that they and any company they use to process credit card authorizations have the correct information on file.
If you get a message about needing to contact iTunes support then you can do so here : http://www.apple.com/support/itunes/contact/