Why is the jquery script not working?

This worked for me:

<script>
     jQuery.noConflict();
     // Use jQuery via jQuery() instead of via $()
     jQuery(document).ready(function(){
       jQuery("div").hide();
     });  
</script>

Reason: "Many JavaScript libraries use $ as a function or variable name, just as jQuery does. In jQuery's case, $ is just an alias for jQuery, so all functionality is available without using $".

Read full reason here: https://api.jquery.com/jquery.noconflict/

If this solves your issue, it's likely another library is also using $.

Samuel Liew is right. sometimes jquery conflict with the other jqueries. to solve this problem you need to put them in such a order that they may not conflict with each other. do one thing: open your application in google chrome and inspect bottom right corner with red marked errors. which kind of error that is?