High Sierra Password Issue

That means you have lost your account. This may help: OS X Mavericks- Restore a deleted user. The following is a bit old so I'm not sure it still works:

How to Create a New User Account if You Cannot Access Your Admin Account

1. Reboot the computer. At the chime press and hold down the COMMAND-S keys to start into Single-user Mode.
2. You will boot to a black screen with scrolling white text, release the keys and wait until it finishes.
3. Enter the following lines at the command prompt pressing RETURN after each:

mount -uw /

rm /var/db/.applesetupdone

reboot

1. After you reboot the computer you will be taken to the Setup Assistant. When you get to the point where you are asked to set up your new user account, create a new admin account. Be sure to name this user account something different than the admin user account that already exists on the system. When you finish the Setup Assistant, it will automatically log you into the new account.
2. You can transfer your files from the old account to the new one. See Transferring files from one User Account to another. Once you have moved all your files into your new account, you can delete the old account using Accounts or Users & Groups preferences.