If you've travelled (on a subscription) on credit you can request a refund on uitcheckgemist.nl. It might take some time before your missed check out appears on uitcheckgemist.nl and your refund will be added to the balance of your card. You will need to pick this refunded credit up at a machine. 

You could also use Mijn NS, if you have your OV-chipkaart added to a My NS account. This is possible after 24 hours and NS will send the refund to your bank account.

If you've travelled using a NS Flex subscription, you can request your refund via Mijn NS and there will be an adjustment on your monthly invoice.

OV Pay is the website you can use in case of a missed check out using a debit or credit card.

Forgot to check in or out | Customer service | NS