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You can follow 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

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.”

Additional resources:

• Connect to SQL Server When System Administrators Are Locked Out
• Leveraging Service SIDs to Logon to SQL Server 2012 and SQL Server 2014 Instances with Sysadmin Privileges
• Recover access to a SQL Server instance using PsExec

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).