I forgot my "sa" password and lost administrator account so I have started sql server using single user mode as below
1. Open sql server configuration manager
2. Added ;-m as a last parameter in startup parameters in advanced tab
3. Restarted the service
4. Start - run - cmd -
5. SQLCMD -SServerName
Then I got the below error message
Login failed for user Reason: Server is in single user mode. Only one administrator can connect at this time.
To fix the issue I have modified -m to -mSQLCMD in step 2 and it got fixed
After login into SQLCMD then issue the below commands to create new login
create login kalyansa with password='password1'
create login [domain\kalyan] from windows;
You need to remember below points
1. Stop the SQL Server Agent
2. Make sure no other account is logged into SQL Server