Locker works on the basis of zero-knowledge architecture to ensure you are the ONLY person to know your master password. Therefore, Locker has no way to retrieve your master password if you forget it.
The only option is to reset your Locker account and start creating your vault from scratch again. However, you can try some following tips before resorting to the last straw.
Try logging in using another device. You might have logged in on the Locker mobile app or browser extension, which may still remain your session. Once accessed, you can see an item named Locker Master Password. Open it to see your Master Password.
Get a master password hint. If you previously set up a master password hint, Locker will send it to your email inbox once requested.
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Note: A Master Password hint is a reminder that will help you remember your Master Password. Setting up a hint is optional, but we strongly recommend one as it will be useful when you forget your Master Password.
If you are a Premium user who has set up Emergency access, you can regain access to your vault via your trusted emergency contact.
If your trusted contact was granted as Viewer, they can view items in your Vault.
If the trusted contact was granted as Takeover, they can reset your master password.
In case none of this works, you may have to delete your account and register a new one.