If at one point you ever logged into the game successfully, created a character, and started playing, inside your "World of Warcraft Classic" folder there's a folder labeled "WTF." Open your WTF folder and there should be another folder labeled "Account." Inside this Account folder is a list of all the accounts that ever logged in on your computer.
If you've forgotten your password, you'll have to take the account name and send a private message to one of the administrators (Viper or Daemon) with as much account information as possible to have them reset the password.
UPDATE: The new
password reset feature is now live. If you're still getting errors during login, try resetting the password using the e-mail address you registered your account with. Make sure it's less than 16 characters in length.