Don't worry! It happens sometimes. Transactions can be declined for several reasons. Sometimes you can guess why it was declined from the response code, but only your credit card issuer or financial institution can confirm the specific reason.
The usual culprits are:
Incorrect credit card number or expiration date
If you are outside the US, some credit card companies will reject international charges
The customer's bank or credit card company was having technical issues when the order was processed
If you made a large number of online purchases within a short period of time, some banks will reject the charges as a fraud prevention measure
If your order is rejected and it happens several times in a row, try clicking the “retry” button again the next day. If there's still no luck, give your financial institution a call.