If you install the Microsoft SQL Server using Windows Authentication mode, the "sa" account is disabled by default. So if you plan to use SQL Server Authentication, you have to enable the "sa" account. This tutorial tells you how to enable the "sa" account.
1. First, Login to the SQL Server Management Studio using Windows Authentication. Right-click on the database instance, and go to Properties.
2. Then on Properties page, click on Security and select SQL Server and Windows Authentication mode, and click on OK to close the Server Properties page.
3. Now you will get dialog box saying that you should restart your SQL Server to take the changes take effect. Is is not done yet, you have to done one more thing to enable the "sa" login.
4. Now expand Security folder and go to Logins. You can see the "sa" account is disabled when you install SQL Server using Windows Authentication mode.
5. Then right-click on the "sa" account and go to Login Properties. There you can set a password for the "sa" account.
6. Click on the Status page. There you can see the "sa" account is disabled by default. Click on the Enabled button to enable it. Then click on Ok to close the "sa" Login Properties.
Now "sa" account is enabled and you can login to the SQL instance using the "sa" account after restarting the SQL Server.
Note: SQL Server service needs to be restarted to make this change effective.