Help with technical issues please

my friend and i have been working on a server for about a month now, and we put so much effort into it! he had to reset his chrome after an unfortunate line of events, and now he can't login! whenever he tries to sign it, it says "this username does not exist". when he tries to create a new account with the same name, it says that the username is taken! his account's name is exampleabuser.