If you are using your YubiKey with a service or application, the policy for lost or stolen YubiKeys is dependent on the service/application and their account recovery process.
If you have a spare key added to your account, or if you have any other means of authentication activated, then you should easily be able to regain access to your account by using either method. When you have logged in to the concerned account(s), make sure to remove the lost YubiKey as an authentication alternative. We recommend that you register a new YubiKey right away.
If you’d want to know more about spare keys, kindly see the article mentioned above.
Please note that it's not Yubico who implements the support of security key(s) for all the different services/applications. The options for authentication, and how it is setup in that environment, is decided by the service/application. Therefore if account recovery is needed, you will be required to contact said service directly for support.
I found my lost YubiKey, is it safe to use?
If your YubiKey was lost and recovered in a place you are familiar with (your home, home of a friend/relative), then it should be safe to use. If it was lost in a public place and later recovered, it may be best to reset it.
Before resetting your YubiKey make sure that you access all of the services/applications you already protect with the YubiKey by using an alternate method (spare YubiKey, recovery codes, etc.), and remove the previously lost YubiKey as part of the two factor authentication flow prior to resetting it.
Be advised! - resetting the YubiKey will effectively unregister the key with every account it has been registered with using FIDO U2F or FIDO2.
The steps for resetting the YubiKey can be found here: How to reset the YubiKey back to factory defaults
Can I deactivate a lost YubiKey?
There is no way to disable your YubiKey remotely. However, there are some safeguards that can be taken depending on how you’re using the key, such as setting a PIN to protect the Yubico Authenticator from being accessed unwillingly, or setting up a FIDO2 PIN on your YubiKey as a safeguard.
Ultimately, we at Yubico always recommend having more than one YubiKey, in case your primary YubiKey is misplaced or stolen. That way you would still be able to authenticate to your account using your spare key. You likely have at least one spare key to your house, car and other things you hold close to your heart, so why wouldn’t you want a spare key for your YubiKey to protect your accounts?
I don’t have a spare key setup, how can I access my account?
If you do not have an additional YubiKey added, it is recommended to have another form of 2FA added to your accounts. In either case, you should be able to get access to your account and remove the lost key from the account. Please note that if a spare key or another authentication method hasn’t been added, you will need to contact the service/website for help with recovering your account.