Administrator problem

I wanted to change the name of the administrator on my iMac (OS high Sierra) I went into users and groups, clicked on my administrator, right clicked and a box came up with the admin details. I changed the user name and clicked ok.

Now when I click on user accounts I click on the padlock the enter name and pass word is blank, I put in new name and usual password but it will to recognise me or password. Can anybody tell me how to restore my previous admin name and password.