In Web.config file

string str = ConfigurationManager.ConnectionStrings["sqlconstr"].ToString();

string str1 = ConfigurationManager.AppSettings[0].ToString();

or

string str1 = ConfigurationManager.AppSettings["sqlconstrusingappset"].ToString();

//connection string using ConnectionString

con = new SqlConnection(str);

or

//connection string using AppSettings

con = new SqlConnection(str1);

con.Open();