[NullReferenceException: Object reference not set to an instance of an object.]

because detail is null.  You are missing some line assigning a value for example detail = new Detail();

From message sent direct to me:

 I already assigned above!! But there is still an error :(

//get selected event's data
        if (!String.IsNullOrWhiteSpace(Request.QueryString["id"]))
        {
            int id = Convert.ToInt32(Request.QueryString["id"]);

            EventDetail Detail = new EventDetail();
            EventDetail detail = Detail.getEventDetail(id);
   
//fill pg w data
            lblPrice.Text = "Price per unit: <br/>$ " + detail.eventPrice;
            lblTitle.Text = detail.eventDetailName;
            lblDescription.Text = detail.eventDesc;
            lblItemNr.Text = id.ToString();

if (!String.IsNullOrWhiteSpace(Request.QueryString["id"]))
        {
            int id = Convert.ToInt32(Request.QueryString["id"]);

            EventDetail Detail = new EventDetail();
            EventDetail detail = Detail.getEventDetail(id);
 if(detail == null)
{
  MessageBox.Show("No event found");
  return;
}

//fill pg w data
            lblPrice.Text = "Price per unit: <br/>$ " + detail.eventPrice;
            lblTitle.Text = detail.eventDetailName;
            lblDescription.Text = detail.eventDesc;
            lblItemNr.Text = id.ToString();

Here is the getEventDetail mthod in Class EventDetail if it helps

 public EventDetail getEventDetail(int eventdetailid)
    {
        EventDetail eveDetail = null;

        int eventtypeid, participants;
        string eventdetailname, eventdate, eventdesc, location, starttime, endtime, eventimage;
        decimal eventprice;

        string queryStr = "SELECT * FROM EventDetail WHERE EventDetail_ID = @EventDetail_ID";

        SqlConnection conn = new SqlConnection(_connStr);
        SqlCommand cmd = new SqlCommand(queryStr, conn);
        cmd.Parameters.AddWithValue("@EventDetail_ID", EventDetail_ID);

        conn.Open();
        SqlDataReader dr = cmd.ExecuteReader();

        if (dr.Read())
        {
            eventtypeid = int.Parse(dr["EventTypeID"].ToString());
            eventdetailname = dr["eventDetailName"].ToString();
            eventdate = dr["eventDate"].ToString();
            eventprice = decimal.Parse(dr["eventPrice"].ToString());
            eventdesc = dr["eventDesc"].ToString();
            location = dr["eventLocation"].ToString();
            starttime = dr["startTime"].ToString();
            endtime = dr["endTime"].ToString();
            eventimage = dr["eventImage"].ToString();
            participants = int.Parse(dr["eventParticipants"].ToString());

            eveDetail = new EventDetail(eventdetailid, eventtypeid, eventdetailname, eventdate, eventprice, eventdesc, location, starttime, endtime, eventimage, participants);
        }
        else { eveDetail = null; }

        conn.Close();
        dr.Close();
        dr.Dispose();

        return eveDetail;
    }

public EventDetail getEventDetail(int eventdetailid)
....
cmd.Parameters.AddWithValue("@EventDetail_ID", EventDetail_ID);

You pass in an int value (eventdetailid) into the function then use a totally different value (EventDetail_ID) as a parameter.  I expect there is no event with the ID you actually use to filter the data source, so your function returns null and then you get the error you posted.

It works now. Thankyou !!!x