In SQL Server 2008 R2 - why can't I change the sa credentails

Question

I'm using AMAZON cloud with SQL Server 2008 R2 installed. I tried to create a password to sa and saved it. But when I went back to SQL Server (after I closed SQL Server and opened it again) it does not let me enter it with the sa credentials that I sat. (I can gain access using Windows Authentication) , stating

Login failed for user 'sa'. (.Net SqlClient Data Provider)

The error number is 18456. I can go in only with Windows Authentication. How can I set sa properly ? Any Ideas?

Here is the full text:

Cannot connect to [server name].

Login failed for user 'sa'. (.Net SqlClient Data Provider)

Server Name: [server name]
Error Number: 18456
Severity: 14
State: 1
Line Number: 65536

Answer 1

It should have also given you a state code as well; searching the combination will show you exactly what the issue is. Are you sure you enabled mixed mode authentication when installing? This article shows you how to enable it: SQL 2008 R2: Enabling Mixed Mode Authentication

This code will verify whether it has been configured for mixed mode authentication already:

SELECT CASE SERVERPROPERTY('IsIntegratedSecurityOnly')   
WHEN 1 THEN 'Windows Authentication'   
WHEN 0 THEN 'Windows and SQL Server Authentication'   
END as [Authentication Mode]  

Answer 2

I finally got it right. First I followed "SQL 2008 R2: Enabling Mixed Mode Authentication" link that Inspired has provided me. Second I opended the EC2 CONSOLE, choosed my Instance and choosed "action" -> "reboot". that's it. Thank you Inspired for your help.