Error-Object reference is required for non static field method

Question

Hi

   On hdfBookId.Value , hdfMobile i am getting above error

public static void getPopUp()
        {

                    BALBooks bALBooks = new BALBooks();
                    BALStudents bALStudents = new BALStudents();
                    StudentDetail Result = bALStudents.GetStudentDetailByLoginID(bALStudents.GetStudentIDByMobile(hdfMobile.Value));


                    BindData(Convert.ToInt32(hdfBookId.Value), Result.LoginCode);

        }

Thanks