To start SQL Server in single-user mode add the parameter -m at the command line. The easiest way to do this is to use Configuration Manager. Stop the SQL Server Instance you want to change. Right click the instance to open the Properties dialog and click the Startup Parameters tab. Enter –m in the Startup parameters.
Next, start the SQL Server Instance. Then open an elevated command prompt and enter sqlcmd. In the sqlcmd windows enter a command like you see following to add your login to the sysadmin group.
EXEC sp_addsrvrolemember ‚CONTOSO\mikeo2′, ’sysadmin‘;
GO