Although this thread is for Westminster New, I see you are using Winchester Responsive - no problem because, as I recall, the two are basically the same template under the hood. I have used Westminster New, not necessarily the latest version of the template but hopefully the detail below matches yours somewhat.
The source code of your site doesn't contain any reference to the menu, so it's not hidden by css, instead there must be something missing in your template files.
The hamburger menu doesn't have a switch that controls it, instead it is loaded by javascript in
includes/templates/winchester_responsive/common/tpl_main_page.php
in that file you should have something like:
Code:
<?php if ($detect->isMobile() && !$detect->isTablet() or $detect->isMobile() && !$detect->isTablet() && $_SESSION['display_mode']=='isMobile' or $detect->isTablet() && $_SESSION['display_mode']=='isMobile' or $_SESSION['display_mode']=='isMobile') { ?>
<script type="text/javascript">
$('ul.slimmenu').slimmenu(
{
resizeWidth: '800',
collapserTitle: '',
animSpeed: 'medium',
easingEffect: null,
indentChildren: false,
childrenIndenter: ' '
});
</script>
(
as an aside the content of the h/burger menu is subsequently controlled by the file
includes/templates/winchester_responsive/templates/tpl_modules_mobile_categories_tabs.php
)
You should check your template's tpl_main_page.php with the original source file.
Failing that, verify that all of the template's source files have been correctly uploaded - if they are then I can't see any reason why the menu wouldn't show.