A declined charge can happen for a number of reasons — and it doesn't necessarily mean something is wrong with your card.
When a charge is submitted, your bank's automated systems evaluate whether to approve it based on signals like your spending habits, account balance, expiration date, and CVC. These systems are constantly changing, so even a card that's worked before can be flagged by your bank's fraud detection.
What can I do?
The best next step is to call your bank directly (their number is on the back of your card). They can look into what triggered the decline and, in many cases, clear it so you can try the charge again.