Jquery does not work?

Jquery does not work?

here i write jquery in my asp.net forum but not working please suggest where is i'm wrong?

<script type="text/javascript">
  $(document).ready(function() {
    $('#ddlPaymentMethod').change(function(){
        if($('#ddlPaymentMethod').val() == "")
        {
             $('#ddlService').prop("disabled", true);
        }
        else
        {
             $('#ddlService').prop("disabled", false);
        }
    });
    $("#ddlService").change(function(){
        if($('#ddlService').val() == "")
        {
             $('#ddlCounselingMethod').prop("disabled", true);
        }
        else
        {
             $('#ddlCounselingMethod').prop("disabled", false);
        }
    });
});
   
 
</script>