First of all a simple advice if you are a complete beginner: i suggest you start from scratch with a tutorial that step by step takes you to create a theme. That way you will have a better understanding on how it works. Underscores is great i use it my self but if you are a total newbie, it could be more difficult to start with.
Do you have WP_DEBUG set to true in your wp-config.php file? If not go on the main directory of you WordPress install and add
define('WP_DEBUG', true);
It will allow you to see errors and help you identify what is the problem.
Here i reproduced my working page.php using Underscores
<?php
/**
* The template for displaying all pages
*
* This is the template that displays all pages by default.
* Please note that this is the WordPress construct of pages
* and that other 'pages' on your WordPress site may use a
* different template.
*
* @link https://developer.wordpress.org/themes/basics/template-hierarchy/
*
* @package sample
*/
get_header(); ?>
<div id="primary" class="content-area">
<main id="main" class="site-main">
<?php
while ( have_posts() ) : the_post(); ?>
<article id="post-<?php the_ID(); ?>" <?php post_class(); ?>>
<?php // YOUR CONTENT AND TEMPLATE for PAGES GOES HERE ?>
</article><!-- #post-<?php the_ID(); ?> -->
<?php
// get_template_part( 'template-parts/content', 'page' );
// as you can see i just copied the content of content-page.php here
// and commented out the get_template_part function
// you can delete this, it is just to explain a little more ;)
?>
<?php
// If comments are open or we have at least one comment, load up the comment template.
if ( comments_open() || get_comments_number() ) :
comments_template();
endif;
endwhile; // End of the loop.
?>
</main><!-- #main -->
</div><!-- #primary -->
<?php
get_sidebar();
get_footer();