How can system administrator get back access of SQL Server ?

I got solution how system administrator can get back access by the instance of SQL server running in single-user mode.

 

When all windows groups & all login that are members of the sysadmin fixed server role have been removed by mistake. 

 

Please refer detail steps from microsoft: 

 

https://docs.microsoft.com/en-us/sql/database-engine/configure-windows/connect-to-sql-server-when-system-administrators-are-locked-out

I don't believe that sysadmin can be removed from SQL Server.

 

I think, server might using windows authentication only.

 

Do check that by login with Windows authentication and goto server properties