If you are using your YubiKey with a service or application, the policy for lost or stolen YubiKeys depends on how that service or application deals with the situation
Make sure to remove your lost YubiKey as a 2FA method after you regain access to your account.
Question: How do I perform a LogIn if I don’t have my Yubikey
Answer: Use your Backup Code, that you generated when you added your YubiKey for that account. Or use your Backup YubiKey.
If you have already lost your YubiKey, do not panic. Many services have a verification process to allow you to verify your identity to regain access. Note this is not a walk in the park, depending on the service you used.