You can the steps mentioned in the link below to reset the SA password:

* [Disaster Recovery: What to do when the SA account password is lost in SQL Server 2005][1]

Steps summarised below:

> 1. Open SQL Server Configuration Manager from Start Menu > Programs > Microsoft SQL Server 20xx > Configuration Tools > relevant to the newest version of SQL Server you have installed (e.g. if you have 2005 and 2012 installed, use the 2012 version). Don't have a Start Menu? On Windows 8's Start screen, start typing SQL Server Con... until it shows up.
> 2. Stop the SQL Server instance you need to recover by right-clicking the instance in SQL Server Services and selecting "Stop"
> 3. Right-click the instance you just stopped, click Properties, and in the “Advanced” tab, in the Properties text box add “;–m” to the end of the list in the “Startup parameters” option (on newer versions, you can go directly to the "Startup Parameters" tab, type "-m" and click Add, without worrying about the syntax, the semi-colon, or anything else).
> 4. Click the “OK” button, and restart the SQL Server Instance
> 5. After the SQL Server Instance starts in single-user mode, the Windows Administrator account is able to connect to SQL Server using the sqlcmd utility using Windows authentication. You can use Transact-SQL commands such as "sp_addsrvrolemember" to add an existing login (or a newly created one) to the sysadmin server role.
> 
The following example adds the account "Buck" in the "CONTOSO" domain to the sysadmin role:
>
>`EXEC sp_addsrvrolemember 'CONTOSO\Buck', 'sysadmin';`
>
> Once the sysadmin access has been recovered, remove the “;-m” from the startup parameters using the Configuration Manager and restart the SQL Server instance one more time.
>
> **NOTE:** make sure there is no space between “;” and “-m”, the registry
    parameter parser is sensitive to such typos. You should see an entry
    in the SQL Server ERRORLOG file that says “SQL Server started in
    single-user mode.”

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Additional resources:

* [Connect to SQL Server When System Administrators Are Locked Out][2]
* [Leveraging Service SIDs to Logon to SQL Server 2012 and SQL Server 2014 Instances with Sysadmin Privileges][3]
* [Recover access to a SQL Server instance using PsExec][4]

Ultimately, you could always copy the database files to another instance, or even reinstall SQL Server (adding a local account as sysadmin during that process).


  [1]: http://blogs.msdn.com/b/raulga/archive/2007/07/12/disaster-recovery-what-to-do-when-the-sa-account-password-is-lost-in-sql-server-2005.aspx
  [2]: https://msdn.microsoft.com/en-us/library/dd207004.aspx
  [3]: http://sqlblog.com/blogs/argenis_fernandez/archive/2012/01/12/leveraging-service-sids-to-logon-to-sql-server-2012-instances-with-sysadmin-privileges.aspx
  [4]: https://www.mssqltips.com/sqlservertip/2682/recover-access-to-a-sql-server-instance/