With LocalAdmin become sysadmin SQL Server I am a local Admin on my machine.  My account wasn't added when SQL Server was installed, and I don't want to have to reinstall SQL Server.  It worked until I tried to log and run the commands via SQLCMD or SSMS.  I got the error message 


  Login failed for user &lt;Machine_Name&gt;/&lt;Windows Name&gt;. Reason: Server is in single user mode. Only one administrator can connect at this time...


I found this blog and added the application name after the m switch.  But no dice, I still get the above error.  The User Account looks fine I guess, I am an Administrator. 

Also, 
  I had to start the cmd prompt as an Admin to start sqlserver in single user mode, or else I couldn't have access to the log files that the instance was using and it was giving me an error to that.  So I ran the cmd prompt as admin. But still, I am the same admin trying to run SQLCMD or SSMS.  It shouldn't be telling me there is already an admin logged in, because I am that admin.  What am I missing?

This is on SQLExpress.
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: