protected void btnDevam_Click(object sender, EventArgs 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!"; 
        }

    }