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 on your Windows:
Log in to the application and in the menu there will be a Forgot Master Password option - click on it.
After that type in the Recovery Key, which is given to you during the registration, click Continue and set your new Master Password.
To reset your Master Password on your macOS:
To get a prompt to change your Master Password enter an incorrect password.
Once you do that, click on the Forgot password.. option, which will open a new window to enter the Recovery Key, which is given to you during the registration, click Enter, and set your new Master Password.
Unfortunately, if you do not have the Recovery Key stored anywhere, there is nothing that can be done to access your vault as we are unable to reset your account.