using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;
using System.Web.Configuration;

public partial class login : System.Web.UI.Page
{
    SqlConnection con;
    string baglanti = WebConfigurationManager.ConnectionStrings["LocalSqlServer"].ConnectionString;
    protected void Page_Load(object sender, EventArgs e)
    {

    }

    protected void btnDevam_Click(object sender, ImageClickEventArgs e)
    {
        if (txtSifre.Text == "" || txtEmail.Text == "")
        {
            lblOlumsuz.Text = "E-Mail adresi veya şifre hatalı!"; 
        }
        else
        {
            con = new SqlConnection(baglanti);
            SqlCommand com = new SqlCommand("select email from members where email=@email and sifre=@sifre", con);
            com.Parameters.AddWithValue("@email", txtEmail.Text);
            com.Parameters.AddWithValue("@sifre", md5.MD5Yap(txtSifre.Text.ToLower()));
            if (com.Connection.State == ConnectionState.Closed)
                com.Connection.Open();
            if (com.ExecuteScalar() != null)
                Response.Redirect("default.aspx");
            else
                lblOlumsuz.Text = "E-Mail adresi veya şifre hatalı!"; 
        }
    }
}

valla .cs dosyası böyle. btnDevam_Click'e tıklandığında txtSifre.Text ve txtEmail.Text her zaman boş dönüyor.Girilen değerler boş dönüyor hep.