Loss Recovery Key

If you have another trusted Apple device that is logged into your iCloud account, you can generate a new recovery key. Otherwise, you'll need to find the key or you will never regain access to your account.

How to generate a recovery key - Apple Support

From the article:

Using a recovery key is more secure, but it means that you’re responsible for maintaining access to your trusted devices and your recovery key. If you lose both of these items, you could be locked out of your account permanently. With that in mind, it's important to keep your recovery key in a safe place. You might want to give a copy of your recovery key to a family member, or keep copies in more than one place. That way you always have your recovery key when you need it.