How do I grant myself admin access to a local SQL Server instance?

Open a command prompt window. If you have a default instance of SQL Server already running, run the following command on the command prompt to stop the SQL Server service:

net stop mssqlserver

Now go to the directory where SQL server is installed. The directory can for instance be one of these:

C:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\BinnC:\Program Files\Microsoft SQL Server\MSSQL13.MSSQLSERVER\MSSQL\Binn

Figure out your MSSQL directory and CD into it as such:

CD C:\Program Files\Microsoft SQL Server\MSSQL10_50.MSSQLSERVER\MSSQL\Binn

Now run the following command to start SQL Server in single user mode. As SQLCMD is being specified, only one SQLCMD connection can be made (from another command prompt window).

sqlservr -m"SQLCMD"

Now, open another command prompt window as the same user as the one that started SQL Server in single user mode above, and in it, run:

sqlcmd

And press enter. Now you can execute SQL statements against the SQL Server instance running in single user mode:

create login [<<DOMAIN\USERNAME>>] from windows;-- For older versions of SQL Server:EXEC sys.sp_addsrvrolemember @loginame = N'<<DOMAIN\USERNAME>>', @rolename = N'sysadmin';-- For newer versions of SQL Server:ALTER SERVER ROLE [sysadmin] ADD MEMBER [<<DOMAIN\USERNAME>>];GO

Source.

UPDATEDDo not forget a semicolon after ALTER SERVER ROLE [sysadmin] ADD MEMBER [<<DOMAIN\USERNAME>>]; and do not add extra semicolon after GO or the command never executes.

Yes - it appears you forgot to add yourself to the sysadmin role when installing SQL Server. If you are a local administrator on your machine, this blog post can help you use SQLCMD to get your account into the SQL Server sysadmin group without having to reinstall. It's a bit of a security hole in SQL Server, if you ask me, but it'll help you out in this case.

I adopted a SQL 2012 database where I was not a sysadmin but was an administrator on the machine. I used SSMS with "Run as Administrator", added my NT account as a SQL login and set the server role to sysadmin. No problem.