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.

Hi! I've go to the same problem here! I upgraded my MacBook yesterday and it took like all day, so when the afternoon came along, the process finished, I was turning on my laptop when it did ask me for my password, I pressed enter and then, there was a blurry screen with two forms to fill in "username " and " password " and there was no picture on it " it seemed like a guest session but I cannot login either ! I panicked and stressed out because I have important files in my MacBook and I don't know what else to do. I tried command plus R and it didn't work out. It doesn't seem to read my password or name, funny because I remember it

Forgot Your Account Password

For Lion or later

Reset a Mac OS X 10.7 Lion Password

OS X Mountain Lion- Reset a login password,

OS X Mavericks- Solve password problems,

OS X Yosemite- Reset a login password.

El Capitan Password Reset.

macOS Sierra-Reset a login password.

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

Bypass and reset the password on any Mac in no time - CNET.

If they don't work then you need to reset or change them, I would imagine. The apparent solutions are in Bypass and reset the password on any Mac in no time - CNET and change or reset the password of a macOS user account as listed above.