Can‘t log on to my MscBook (username & password forgotten).

Reset using your Apple ID

In some macOS versions, you can use your Apple ID to reset your login password. At the login screen, keep entering a password until you see a message saying that you can reset your password using Apple ID. If you don't see the message after three attempts, your account isn't set up to allow resetting with Apple ID.

1. Click  next to the Apple ID message, then follow the onscreen instructions to enter your Apple ID and create a new password. You'll be asked to restart when done.
2. Log in with your new password.
3. Determine whether to create a new login keychain.

Reset using another admin account

If you know the name and password of an admin account on your Mac, you can use that account to reset the password. 

1. Log in with the name and password of the other admin account.
2. Choose System Preferences from the Apple menu, then click Users & Groups.
3. Click , then enter the admin name and password again.
4. Select your user name from the list of users.
5. Click the Reset Password button, then follow the onscreen instructions to create a new password:
6. 
7. Choose Log Out from the Apple menu.
8. Log in to your account using your new password.
9. Determine whether to create a new login keychain. 

Reset using the Reset Password assistant (FileVault must be on)

If FileVault is turned on, you might be able to reset your password using the Reset Password assistant:

1. Wait up to a minute at the login screen, until you see a message saying that you can use the power button on your Mac to shut down and start up again in Recovery OS. If you don't see this message, FileVault isn't on.
2. Press and hold the power button until your Mac turns off.
3. Press the power button again to turn on your Mac.
4. When the Reset Password window appears, follow the onscreen instructions to create a new password.
5. If you need to connect to Wi-Fi, move your pointer to the top of the screen and use the Wi-Fi menu  to connect. To exit without resetting your password, choose Apple menu > Restart.
6. 
7. When done, click Restart.
8. If you were able to reset your password with the Reset Password assistant, log in to your account using your new password.
9. Determine whether to create a new login keychain.

Reset using your Recovery Key (FileVault must be on)

If FileVault is turned on and you have a FileVault Recovery Key, you can use that key to reset your password.

1. At the login screen, keep entering a password until you see a message saying that you can reset your password using your Recovery Key. If you don't see the message after three attempts, FileVault isn't on.
2. Click  next to the message. The password field changes to a Recovery Key field.
3. Enter your Recovery Key. Use uppercase characters, and include the hyphens. 
4. Follow the onscreen instructions to create a new password, then click Reset Password when done.
5. Determine whether to create a new login keychain.

Change or reset the password of a macOS user account - Apple Support

https://support.apple.com/en-in/HT202860