Difficult to say why without no information. What happens when you try to buy something ? If you are getting an error message what does it say ?
If you are getting a message to contact iTunes Support, and if you haven't already done as instructed (these are user forums) :
- click on your country's flag (the country where you are, and which is therefore the country on your account)
- click on Contact Support towards the top right
- click on Get Help under 'Talk To Us'
Or use this form : https://www.apple.com/emea/support/itunes/contact.html
If it's a different problem ... ?