This is helpful to you
======================
Include NameSpaces also
using System.Data.SqlClient;
using System.Web.configuration;
String connStr = ConfigurationManager.ConnectionStrings["DATABASENAMEConnectionString"].ConnectionString;
IN WEB.CONFIG File write this
=============================
<connectionstring>
<add name="DatabaseNameConnectionString" connectionstring="Data Source=Servername;Initial Catalog=DatabaseName;User ID=sa; Password=123" providername="System.Data.SqlClient" />
</connectionstring>