We are able to connect to MSSQL Server Management Studio with Windows Authentication but unable to connect with SQL Server Authentication. The 'sa' login details are correct but still getting the following error message:

Login failed for user 'sa'. (Microsoft SQL Server, Error: 18456)

In order to resolve the issue, please refer to the following steps:

1. Login to the MSSQL Server Management Studio with Windows Authentication.
2. In SQL Server Management Studio Object Explorer, right-click the server, and then click Properties.
3. Under the Server Properties, select a page of "Security".
4. Select the Server authentication as "SQL Server and Windows Authentication mode" and click Ok.

5. Restart the SQL Services and then try to login with 'sa' details.

Hope this will help you....