Hi,
Thank you for sharing the file. Your override is of show.php (the main product page template) from version 5.1.1. The only customization is a back button at the very top of the page. Since the rest of the
file is unchanged from 5.1.1, the override was replacing the entire 6.4.0 template with the old version.
To restore your back link, create a new override of show.php via Display > Views and add these lines at the very beginning of the file, before the existing code:
<a onclick="window.history.back(); return false;" href="#" >
<span class="icon-32-back" title="<?php echo JText::_('HIKA_BACK'); ?>"></span>
<?php echo JText::_('HIKA_BACK'); ?>
</a>This way you keep the back link while using the current 6.4.0 template code for everything else.