If your Windows user account doesn’t have access to a database, add the account to the sysadmin role to get permission to access all databases on the server.
First, stop the SQL Express service.
The one thing the article omits (I don’t see it there) is the bold part:
C:\Program Files (x86)\Microsoft SQL Server\MSSQL11.SQLEXPRESS\MSSQL\Binn> .\sqlservr.exe -sSQLEXPRESS -m”SQLCMD” -c
That allows a client with the name “SQLCMD” to connect. If you omit that part and just put -m, you won’t be able to connect:
Sqlcmd: Error: Microsoft SQL Server Native Client 11.0 : Login failed for user ‘DOMAIN\user’. Reason: Server is in single user mode. Only one administrator can connect at this time..
Once you’ve started the server in single user mode, open another window and run:
SQLCMD -S .\SQLEXPRESS
to connect to the SQL Express instance on the local machine.
sp_addsrvrolemember ‘DOMAIN\user’, ‘sysadmin’
Then press Control-C to close sqlcmd and single user mode.