Steps:
1. shut down SQL Server from services
2. open cmd window (as administrator) and run single-user mode as local admin with this command:
"c:\Program Files\Microsoft SQL Server\MSSQL10.SQLEXPRESS\MSSQL\Binn\sqlservr.exe" -m -s SQLEXPRESS
3. open another cmd window (as administrator)
4. open sqlcmd:
sqlcmd -S .\SQLEXPRESS
Now add the sysadmin user:
a. sp_addsrvrolemember 'domain\user', 'sysadmin'
b. GO
5. now Ctrl+C the single-user mode from the first cmd window to kill SQL Server. Now restart it from services the normal way.
6.Exit cmd window, Log into Management Studio and the user you created should be listed under logins with the credential of "sysadmin."
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Another way to add the current user to the sysadmin role in SQL Server follow
