With LocalAdmin become sysadmin SQL Server
For such a situation we have made use of PSExec utility to get regain access on the SQL server.

You can download the utility from here , extract and copy the exe file to the drive location from where you can run below command.

Once downloaded you need to open the cmd with admin and run below:

PsExec -s -i "C:\Program Files (x86)\Microsoft SQL Server\110\Tools\Binn\ManagementStudio\Ssms.exe"


The path above may vary, like for one of our SQL 2k8 , we found the path to be:

"C:\Program Files (x86)\Microsoft SQL Server\100\Tools\Binn\VSShell\Common7\IDE\Ssms.exe"


This will make you connect to the current via sever Windows Authentication, and the username is hard-coded as NT AUTHORITY\SYSTEM.

YOu can create the required login from here: