If the payment was by card check:
1. If the card you are using expired?
2. Are your card details correct, including security (CVC) code and expiry date?
3. Does your billing address on your order match the details held by your card provider?
4. Have you checked your card has sufficient funds?
If you paid via PayPal check:
1) Did you entered your PayPal email address correctly?
2) Does your billing address on your order match the details held by PayPal?
If this doesn’t help, contact your card issuer or bank.