The default user when a new QuickBooks Desktop file is created is the Admin. Try typing "Admin" in the username field. If you haven't entered password previously, leave it blank.
If this doesn't work, I recommend reaching out to our Customer Care Support team for further assistance in recovering your username.
Here's how:
Select QuickBooks Desktop Help from the Help menu.
Click Contact Us.
Type in a short description of your concern, then Continue.
Choose a way to connect with support.
For more details about our support availability, refer to this article: Contact Support.