how could two of my users, including the admin user be missing from my Mac?

If I did not hide the users, how could they be missing? I have one user left on the Mac, and my admin and another user are missing. I can see the folders but it does to recognize my admin user and/or password. I tried to show hidden users in the terminal using the standard user that is left on my Mac, but it does not work. Help, I need my admin user so I can manage the Mac, and access my folders.