If you are locked out of your computer due to Yubico Login for Windows, an unknown or incorrect username could be the issue. This isn't new login credentials, it is the username and password to your PC. Note that, with most configurations, you should not touch your YubiKey when signing in to Windows with Yubico Login for Windows.
If you have only installed Yubico Login for Windows and rebooted, then the YubiKey is not yet required, although the login screen will look different at this point. Once your system has been rebooted following installation, at the login screen, in addition to your password, you will also be required to enter your username. Most likely, you were only required to enter your password beforehand, as this is the default behavior in Windows.
Since it is not common to need to enter your username in Windows, the first thing I would recommend is that you verify what it is.
Since you aren't able to log in to your computer the "normal way," first start your computer in safe mode (see here for instructions). Assuming you are able to successfully log in after booting in safe mode, open the Command Prompt program from the Start menu, and in the window that appears, type
whoami, and press Enter on your keyboard.
The command's output, which should be in the form
DESKTOP-1JJKFIDS\jdoe, will contain your username - it will be the portion following the slash. In this example, the username would be
Once you have this, exit safe mode by rebooting, and try again to log in to your computer. If you did not have to enter a password prior to installing Yubico Login for Windows, leave the password field on the login screen blank.
Please give this all a try, and if it doesn't help you can open a support ticket here. Please provide a photo/screenshot of the login screen.