Cannot find users on login screen - only guest login available
When at the login screen showing just the Guest user hold down Option  or alt and the right arrow key.  It will populate more users if they exist.

If that doesn't work you can boot into either single user mode (guide to boot to single user) or Restore partition and open terminal.app and run this command:

defaults write /Library/Preferences/com.apple.loginwindow SHOWFULLNAME -bool yes


This will change the login window to be the name and password field.  Then you can enter your username.