pages not showing

I’m experimenting with designing my own theme. Just for the sake of learning, so bear with me if I’m a total noob. I’ve checked countless theme tutorials and I dont understand why my page.php (as well as my 404.php) isn’t showing anything. It just says
“Not Found

The requested URL /alexr/sample-page/ was not found on this server.”
This is the code in my page.php

<?php get_header(); ?>

<div id="main_wrapper">
	<div id="main">
		<?php if(have_posts()) : ?><?php while(have_posts()) : the_post(); ?>
			<?php the_title(); ?>
			<?php the_content(); ?>
		<?php endwhile; ?><?php endif; ?>
	</div>
</div>

<?php get_footer(); ?>