Connection String Error (error : 40)

I have installed SQL Server 2012 Express on a Windows Server 2008 Machine. The name of the machine is "THEMACHINE". While installation, I set SQL Server 2012 as "Default Instance". Now I set my connection string like this :

<add name="BooksContext" providerName="System.Data.SqlClient" connectionString="Data Source=THEMACHINE;Initial Catalog=newdb; User ID=sa; Password=123456"/>

But I get this exception :

{"A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: Named Pipes Provider, error: 40 - Could not open a connection to SQL Server)"}

I checked that database is running. Can you tell me what I'm doing wrong?Thanks.

Reference: Creating and Configuring Universal Data Link (.udl) Files

Do following to Test connection and generate connection string,

1) Create query.udl file in the desktop

2) Double click the file to open and do the following