If you have configuration problems that prevent the server from starting, you can start an instance of Microsoft SQL Server by using the minimal configuration startup option. This is the startup option -f. Starting an instance of SQL Server with minimal configuration automatically puts the server in single-user mode.
When you start an instance of SQL Server in minimal configuration mode, note the following:
Only a single user can connect, and the
CHECKPOINTprocess is not executed.
Remote access and read-ahead are disabled.
Startup stored procedures do not run.
tempdbis configured at the smallest possible size.
After the server has been started with minimal configuration, you should change the appropriate server option value or values, stop, and then restart the server.
Use the sqlcmd utility and the dedicated administrator connection (DAC) to connect to SQL Server. If you use a typical connection, stop the SQL Server Agent service before connecting to an instance of SQL Server in minimal configuration mode. Otherwise, the SQL Server Agent service uses the connection, thereby blocking it.
Start, Stop, or Pause the SQL Server Agent Service
Diagnostic Connection for Database Administrators
Server Configuration Options (SQL Server)
Database Engine Service Startup Options