[jQuery] code is ok but page is reloading itself

this is the code
 $('#open_menu').click(function(){
        $(this).next().filter('div').removeClass('hidden');
        alert('dd');
    });
everything is fine until I will press ok (that shows that my code is
ok) on the alert button. then surprisingly the page is reloaded. i
really dont have any errors, I ve changed lib from pack to unpack and
still something is wrong:(