protected void btnDangNhap_Click(object sender, EventArgs e)
    {
        string strcn = ConfigurationManager.ConnectionStrings["ql"].ConnectionString;
        SqlConnection cn = new SqlConnection(strcn);

        string strsl = "select * from tbl_KhachHang where SoDT='" + txtSDT.Text + "' and MatKhau ='" + txtMK.Text + "'";
        SqlDataAdapter da = new SqlDataAdapter(strsl, cn);
        DataTable dt = new DataTable();
        da.Fill(dt);

        if (dt.Rows.Count == 1)
        {
            Session["kh"] = dt;
            Response.Redirect("TrangChu.aspx");
        }
        else
        {
            Label1.Text = "Thong  tin dang nhap sai!";
        }
    }

<div>
        <h2 style="background-color: orange; font-weight: bolder; margin-top: 20px; margin-bottom: 20px; color: #FF0000;">ĐĂNG NHẬP</h2>
        Số điện thoại&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp;<asp:TextBox ID="txtSDT" runat="server" style="margin-left: 0px" Width="151px"></asp:TextBox>
        &nbsp;<br />
        <br />
        Mật khẩu&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; &nbsp; <asp:TextBox ID="txtMK" runat="server" Width="151px" TextMode="Password"></asp:TextBox>
        <br />
        <br />
        <asp:Label ID="Label1" runat="server" Text="Label" ForeColor="#FF3300"></asp:Label>
        <br />
        <asp:Button CssClass="login" ID="btnDangNhap" runat="server" Text="Đăng nhập" BackColor="#FF9933" OnClick="btnDangNhap_Click"/>
        <br />
        <br />
    </div>

.login {
    margin-left: 150px;
}