No function in GridView.SelectedIndexChanged Event

using System;
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;
using System.IO;
using System.Net;
using System.Net.Mail;

namespace PS
{
    public partial class AdminHome : System.Web.UI.Page
    {
        public SqlConnection cn = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=C:\Users\Bala\MyStuffs\system\PS\PS\App_Data\Database1.mdf;Integrated Security=True;User Instance=True");

        protected void Page_Load(object sender, EventArgs e)
        {
            
        }
        
        protected void GridView1_SelectedIndexChanged(object sender, System.EventArgs e)
        {

            GridViewRow row = GridView1.SelectedRow;
            lblFeedbackNo.Text = row.Cells[1].Text;
            txtComment.Text = row.Cells[2].Text;
            Panel1.Visible = true;
            GridView1.Visible = false;


        }
       

        

        

        protected void btnSubmit_Click(object sender, EventArgs e)
        {
          
	        string aTo = null;
	        string aFrom = "ps_fyp@outlook.com";
	        string aSubject = "WPEHMS Reply Message";
	        string aMsg = "Dear Student," + Environment.NewLine + txtReply.Text + Environment.NewLine + Environment.NewLine + Environment.NewLine + "Regards, " + Environment.NewLine + "WPEHMS";
	if (string.IsNullOrEmpty(txtReply.Text)) {
		lblValidation.Visible = true;

	} else {
		//------------------------------------------------------ Get email from database -------------------------------------------------------
        SqlCommand cmd = new SqlCommand("SELECT fdk_Email From tblFeedback Where fdk_No ='" + lblFeedbackNo.Text + "'",cn);
		cn.Open();
       
        aTo=(string)cmd.ExecuteScalar();
        cn.Close();
        


		//------------------------------------------------- Email service (hotmail) -------------------------------------------------------------
		string mMailServer = "smtp.live.com";
		//this is the name of mail server that email is sent to
		int mPort = 587;
		//port of mail server "use a value of 0 if none need be specified

		MailMessage message = new MailMessage();
		message.From = new MailAddress(aFrom);
		message.Subject = aSubject;
		message.Body = aMsg;
		message.To.Add(aTo);
		message.IsBodyHtml = false;
		message.Priority = MailPriority.Normal;

		SmtpClient oSMTP = new SmtpClient(mMailServer, mPort);
		// creates instance of mail client
		var _with1 = oSMTP;
		_with1.EnableSsl = true;
		_with1.DeliveryMethod = SmtpDeliveryMethod.Network;
		_with1.Credentials = new NetworkCredential("ps_fyp@outlook.com", "Thepassword");

		try {
			oSMTP.Send(message);
		} catch (Exception ex) {
        System.Windows.Forms.MessageBox.Show(ex.Message);
		}

		//
		//------------------------------------------------ Store replied data into database -----------------------------------------------------
		//string theStatus = "Replied";
        SqlCommand cmd1 = new SqlCommand("UPDATE  tblFeedback " + "SET staff_ID = '" + Session["idcode"] + "', fdk_Reply= '" + txtReply.Text + "'  WHERE fdk_No = '" + lblFeedbackNo.Text + "'",cn);
		
		cn.Open();
		cmd1.ExecuteNonQuery();
		cn.Close();
        System.Windows.Forms.MessageBox.Show("Reply Sent Successful");
		Panel1.Visible = false;
		GridView1.DataBind();
		GridView1.Visible = true;
	}
}
        

        
    }
}