Source: How to recover SA password on Microsoft SQL Server 2008 R2 | Gert Van Gorp
OK this little process saved me. Big thanks to Gert for posting this article. If you have local admin access to the SQL server, you can do this procedure which works like a charm.
To log in using sqlcmd to an instance of SQL, make sure to use the following format:
sqlcmd -S ComputerA\instanceB