When a card payment is declined, the reason is shown in the payment's activity log: an expired card, an incorrect security code, insufficient funds, and so on.
Sometimes the customer's bank declines without giving a specific reason. In the activity log this shows as "Payment authorisation declined", and only the bank knows the underlying cause, so the fastest answer is for your customer to check with their card issuer. Their payment link stays active in the meantime, so they can retry the card or pay another way, such as pay by bank.