This passage introduces how to access SQL Server in Single User Mode, when cannot access SQL Server using Windows Authentication Mode.
Steps to set SQL Server access in Single-User Mode:
Step 1: Launch SQL Server Configuration Manager and stop SQL Server instance.
Step 2: Right-click on SQL Server instance and select Properties.
Step 3: Set SQL Server startup mode in Properties window.
Step 4: Save changes and restart the SQL Server instance. This will start SQL Server in single-user mode.
Once you could access SQL Server in single-user mode, new login can be created and added to SA server role with command prompt. So login SQL Server successfully again with SA account will be no problematic even after SA password forgot and Windows authentication is unavailable.
But if you cannot access SQL Server in single-user mode or cannot log on Windows as administrator, the startup setting would not do help you. In this situation, you would need to use SQL Password Genius which could easily unlock SA account or reset SA password offline.
Password Recovery Software