The default username is "admin". You may have been trying username you usually use elsewhere and getting an error because of that. When you reset notebook admin user remains as the default user and the password is reset. However, if you want to create a new user regardless use the following commands at the terminal
from sagenb.notebook.notebook import load_notebook
nb = load_notebook("directory_to_run_sage_in")
user_manager = nb.user_manager()
user_manager.set_accounts(True)
user_manager.add_user("username", "password", "email@place", "user")
nb.save()
What service are you using? http://Sagenb.org?https://cloud.sagemath.com? Your own local copy of Sage and the sage notebook?
The first, I think. Been using it all along, downloaded new version of sage and am now messed up completely. Am almost completely ignorant about IT.