Hi, I'm John! I will help you with this.
Was it directly from the Microsoft Store or from another retailer?
If it was through the Microsoft Store, please check your purchase history in your account, you might find the code listed there. If it was from a retailer, they should be able to provide the redeem code or clarify what happened with your purchase.
You can also refer to this Microsoft guide on redeeming prepaid codes: https://support.xbox.com/help/subscriptions-billing/redeem-codes-gifting/redeem-prepaid-codes
If the issue persists, please provide more details by answering my questions.
Click "Add comment" below to let me know.
Regards,
John J.D.