Check the following:
Is your card still valid?
Have you filled in the following information correctly (card number, expiry date, CVV)?
Check with your bank to make sure you haven't reached your limit, or that they haven't blocked the payment for security reasons.
Does the billing address you have entered match the one registered with your bank? An incorrect address may result in a refusal of payment.
Make sure your card is authorized for international payments. If necessary, contact your bank to activate this feature on your card.
Try changing your browser, sometimes this can fix the problem.