Checking for user and password

I have a login page that checks if user/password exists in my 'users' database. The code I'm using doesn't work even when I enter values that I know to be correct.
 This is what I have:

using System;
using System.Configuration;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;

public partial class LogInPage : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{

}
protected void Login_Authenticate( Go to the complete details ...