In case of master password loss, you would have to use a recovery key, which is given upon registration to the NordLocker after you create a master password.
To reset your master password, log in to the application and when the window appears to write down your master password, click on "Recover it" at the bottom of the application.
After that write down the recovery key, which is given to you during the registration, click "Continue" and set your new master password.
If you have lost your recovery key, you may reset it by logging in to the application, clicking on Account → Reset recovery key.
If you experience any further difficulties with the master password or recovery key, you may contact our customer support team via firstname.lastname@example.org.