Fatal error: Call to undefined function get_header() in (…)

Hi guys, my installation of WordPress has been running perfectly, but all of a sudden it started going crazy, if you will. At first I got 505 errors for the website (not the admin) although the database info was all correct. Then it got itself somehow working, and ever since I’ve been getting the following error whenever I try to access my website (admin still works perfectly):

Fatal error: Call to undefined function get_header() in /hermes/bosnaweb06a/b1538/ipg.erezhenya/glober.co.il/blog/index.php on line 1

Here’s the index.php content:

<?php get_header(); ?>

	<main role="main">
		<!-- section -->
		<section>

			<h1><?php _e( 'Latest Posts', 'html5blank' ); ?></h1>

			<?php get_template_part('loop'); ?>

			<?php get_template_part('pagination'); ?>

		</section>
		<!-- /section -->
	</main>

<?php get_sidebar(); ?>

<?php get_footer(); ?>

Template is html5blank. I tried switching to another template (say 2015), but the same error remains. Strange thing is that I also get an error line at my wp-admin, right under the top menu, which says:

Notice: Trying to get property of non-object in /hermes/bosnaweb06a/b1538/ipg.erezhenya/glober.co.il/blog/wp-content/themes/html5blank/functions.php on line 254

That line 254 in functions.php is:

return '... <a class="view-article" href="' . get_permalink($post->ID) . '">' . __('View Article', 'html5blank') . '</a>';

Website URL is: http://www.glober.co.il/blog/

What’s going on? your help is greatly appreciated.