Login failed for user reason single user mode only one administrator can connect at this time
Microsoft SQL server error 18461
You might get a situation that no sysadmin access to SQL server and server installed by someone. Breaking SQL server and adding SA sysadmin of yourself by SQLCMD. You should be OS administrator.
First find out ‘Binn’ folder where it is installed C or D drive. Better search in my computer.
Open a CMD with administrator
Type: cd /d E:\Program Files\Microsoft SQL Server\130\Tools\Binn
1.Stop the SQL server service
2.Start the service in Single user mode by adding ‘;-mSQLCMD’ to the parameters from SQL server configuration manager — right click SQL service — property — Advance — Startup parameters.
Type: SQLCMD -S SQLserver name -E
ex: sqlcmd -S.\Instance name
CREATE LOGIN [domain\user] FROM WINDOWS WITH DEFAULT_DATABASE=[master]
ALTER SERVER ROLE [sysadmin] ADD MEMBER [domain\user]