I changed my admin user name and now my mac won't accept any name or password for admin changes.

I use high sierra; I changed my administrator username, clicked the lock and then restarted my computer. It no longer shows a user name in that box and doesn't accept any username or password that I put in, even though I can still log into the computer under my original username and password. I'm no longer recognized as an administrator or technically a user...HELP!!!