The only thing we really know is that the credit card declined the charge.  They do not provide any more information than this.  It can be for any of the following reasons:

1) You typed a number wrong.

2) You chose an incorrect expiration date.

3) There is some kind of a fraud hold on the card.  This happens sometimes when the credit card company sees a transaction they don’t believe fits your normal buying pattern and they are worried that it might be fraud.

4) There is some other type of problem with the credit card account. For example, a restriction placed on it by the credit card company that it may not be used outside your country, or insufficient funds available.

With 1, or 2, you can try to submit the order again.  With 3 or 4, you will probably need to call your credit card company and ask them why the charge was declined.

Finally, give them a simple click to contact you that pre-populates the form with their info including what they were purchasing so that they know:
1) You will get enough info to help them
2) They know they don’t have to fill out much and they can trust you will get back to them on their specific problem.