Am having error in gridview edit

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.Security;
using System.Xml.Linq;
using System.Text.RegularExpressions;
using System.Data.SqlClient;
using System.Data;
using System.Configuration;
using System.Drawing;
using System.Web.UI.HtmlControls;
public partial class Hotet_Edit : System.Web.UI.Page
{

    private SqlConnection conn = new SqlConnection("Data Source=.;Initial Catalog=IMANET1;Integrated Security=True");
   // string str = "Data Source=.;Initial Catalog=IMANET1;Integrated Security=True";
    // private SqlConnection con = new SqlConnection("Data Source=.;Initial Catalog=IMANET1;Integrated Security=True");
    protected void Page_Load(object sender, EventArgs e)
    {

         if (!IsPostBack)
            {
                gvbind();
            }
        }
        protected void gvbind()
        {
            conn.Open();
            SqlCommand cmd = new SqlCommand("Select * from HotelBooking", conn);
            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataSet ds = new DataSet();
            da.Fill(ds);
            conn.Close();
            if (ds.Tables[0].Rows.Count > 0)
            {
                GridView1.DataSource = ds;
                GridView1.DataBind();
            }
            else
            {
                ds.Tables[0].Rows.Add(ds.Tables[0].NewRow());
                GridView1.DataSource = ds;
                GridView1.DataBind();
                int columncount = GridView1.Rows[0].Cells.Count;
                GridView1.Rows[0].Cells.Clear();
                GridView1.Rows[0].Cells.Add(new TableCell());
                GridView1.Rows[0].Cells[0].ColumnSpan = columncount;
                GridView1.Rows[0].Cells[0].Text = "No Records Found";
            }
 
        }


        protected void GridView1_RowDeleting(object sender, GridViewDeleteEventArgs e)
        {
            GridViewRow row = (GridViewRow)GridView1.Rows[e.RowIndex];
            Label lbldeleteid = (Label)row.FindControl("lblID");
            conn.Open();
            SqlCommand cmd = new SqlCommand("delete FROM HotelBooking where CustomerID='" + Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value.ToString()) + "'", conn);
            cmd.ExecuteNonQuery();
            conn.Close();
            gvbind();
 
        }
        protected void GridView1_RowEditing(object sender, GridViewEditEventArgs e)
        {
            GridView1.EditIndex = e.NewEditIndex;
            gvbind();
        }
        protected void GridView1_RowUpdating(object sender, GridViewUpdateEventArgs e)
        {
            int CustomerID = Convert.ToInt32(GridView1.DataKeys[e.RowIndex].Value.ToString());
            GridViewRow row = (GridViewRow)GridView1.Rows[e.RowIndex];
            Label lblID = (Label)row.FindControl("lblID");
       
            TextBox txtCustomerID = (TextBox)row.Cells[0].Controls[0];
            TextBox txtRoomNo = (TextBox)row.Cells[0].Controls[0];
            TextBox txtRoomCategory = (TextBox)row.Cells[1].Controls[0];
            TextBox txtTimeCheckedIn = (TextBox)row.Cells[2].Controls[0];
            TextBox txtDay = (TextBox)row.Cells[3].Controls[0];
            TextBox txtMonth = (TextBox)row.Cells[4].Controls[0];
            TextBox txtYear = (TextBox)row.Cells[5].Controls[0];
            TextBox txtName = (TextBox)row.Cells[6].Controls[0];
            TextBox txtGender = (TextBox)row.Cells[7].Controls[0];
            TextBox txtLocation = (TextBox)row.Cells[8].Controls[0];
            TextBox txtPhone = (TextBox)row.Cells[9].Controls[0];
            TextBox txtDailyBookingCost = (TextBox)row.Cells[10].Controls[0];
            TextBox txtDaysBooked = (TextBox)row.Cells[11].Controls[0];
            TextBox txtRecordedBy = (TextBox)row.Cells[12].Controls[0];
            TextBox txtRoomOccupied = (TextBox)row.Cells[13].Controls[0];
            TextBox txtTotal = (TextBox)row.Cells[14].Controls[0];

            GridView1.EditIndex = -1;
            conn.Open();
            //SqlCommand cmd = new SqlCommand("SELECT * FROM detail", conn);
            SqlCommand cmd = new SqlCommand("update HotelBooking set CustomerID='" + txtCustomerID +"',  RoomNo='" + txtRoomNo.Text + "',RoomCategory='" + txtRoomCategory.Text + "', TimeCheckedIn='" + txtTimeCheckedIn.Text + "', Day='" + txtDay.Text + "', Month='" + txtMonth.Text +"', Year='" + txtYear.Text + "', Name='" + txtName.Text + "', Gender='" + txtGender.Text + "', Location='" + txtLocation.Text + "', Phone='" + txtPhone.Text + "',DailyBooking='" + txtDailyBookingCost.Text +"', DaysBooked='" + txtDaysBooked.Text +"',RecordedBy='" + txtRecordedBy.Text +"', RoomOccupied='" + txtRoomOccupied.Text + "', Total='" + txtTotal.Text + " where CustomerID='" + CustomerID + "'", conn);
            cmd.ExecuteNonQuery();
            conn.Close();
            gvbind();
            //GridView1.DataBind();
        }
        protected void GridView1_PageIndexChanging(object sender, GridViewPageEventArgs e)
        {
            GridView1.PageIndex = e.NewPageIndex;
            gvbind();
        }
        protected void GridView1_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
        {
            GridView1.EditIndex = -1;
            gvbind();
        }
        

        }
}