Did you log into an account?
It is possible that some of your payment information needs to be updated or there could be a problem with a balance. You need to go through the steps with Paypal customer service directly.
Did you pay without logging into an account?
Paypal only allows you to charge a lifetime (the credit card and not yours) amount of $750. This prevents major fraud charges from happening. If you paid online without an account and got declined, it's very possible you have reached this maximum. Paypal will not allow you to exceed this maximum. You either need to create an account with them using that credit card or use a completely different credit card that has not reached it's lifetime maximum.