Error: - Object reference not set to an instance of an object.

You never set the InsertCommand ... Try:

        public string AcceptDetails(string ISBN, string DateOrder, string CustName, string CustAddr1,

            string CustAddr2, string CustCity, string CustState, string OrdBy, string OrdStat,

            string CardType, string CardNum)

        {

            string OrderNo;

            string error;

            error = "";

            OrderNo = "1234";

            string InsStr;

            InsStr = "insert into dtorders values " +

                "(@IN, @ON, @DO, @CN, @CA1, @CA2, @CC, @CS, @OB, @ST, @CT, @CNN)";

            try

            {

                SqlConnection conn = new SqlConnection(@"ConnStr");

                SqlCommand cmd1 = new SqlCommand(InsStr, conn);

                SqlDataAdapter dad1 = new SqlDataAdapter("SELECT * FROM dtOrders", conn);

                dad1.InsertCommand = new SqlCommand(InsStr, conn);

                dad1.InsertCommand.Parameters.Add("@IN", SqlDbType.Char, 10).Value = ISBN;

                dad1.InsertCommand.Parameters.Add("@ON", SqlDbType.Char, 5).Value = OrderNo;

                dad1.InsertCommand.Parameters.Add("@DO", SqlDbType.DateTime, 8).Value = Convert.ToDateTime(DateOrder).Date;

                dad1.InsertCommand.Parameters.Add("@CN", SqlDbType.VarChar, 50).Value = CustName;

                dad1.InsertCommand.Parameters.Add("@CA1", SqlDbType.VarChar, 50).Value = CustAddr1;

                dad1.InsertCommand.Parameters.Add("@CA2", SqlDbType.VarChar, 50).Value = CustAddr2;

                dad1.InsertCommand.Parameters.Add("@CC", SqlDbType.VarChar, 20).Value = CustCity;

                dad1.InsertCommand.Parameters.Add("@CS", SqlDbType.VarChar, 10).Value = CustState;

                dad1.InsertCommand.Parameters.Add("@OB", SqlDbType.VarChar, 50).Value = OrdBy;

                dad1.InsertCommand.Parameters.Add("@ST", SqlDbType.VarChar, 20).Value = OrdStat;

                dad1.InsertCommand.Parameters.Add("@CT", SqlDbType.Char, 10).Value = CardType;

                dad1.InsertCommand.Parameters.Add("@CNN", SqlDbType.VarChar, 20).Value = CardNum;

                if (conn.State == ConnectionState.Closed)

                {

                    conn.Open();

                }

                dad1.InsertCommand.ExecuteNonQuery();

                conn.Close();

            }

            catch (Exception E1)

            {

                error = E1.Message;

            }

            string result;

            if (error.Length != 0)

            {

                result = "Record not Inserted due to the following reason: \n" + error;

            }

            else

            {

                result = "Record Inserted!!";

            }

            return result;

        }