WordPress Meta Checkbox

I have a WordPress based website with the following lines of code in my function.php file:

/**
 * Calls the class on the post edit screen.
 */
function call_someClass() {
    new someClass();
}

if ( is_admin() ) {
    add_action( 'load-post.php', 'call_someClass' );
    add_action( 'load-post-new.php', 'call_someClass' );
}

/** 
 * The Class.
 */
class someClass {

    /**
     * Hook into the appropriate actions when the class is constructed.
     */
    public function __construct() {
        add_action( 'add_meta_boxes', array( $this, 'add_meta_box' ) );
        add_action( 'save_post', array( $this, 'save' ) );
    }

    /**
     * Adds the meta box container.
     */
    public function add_meta_box( $post_type ) {
            $post_types = array('post', 'page');     //limit meta box to certain post types
            if ( in_array( $post_type, $post_types )) {
                add_meta_box(
                    'some_meta_box_name'
                    ,__( 'Display Call-To-Action Sticky', 'myplugin_textdomain' )
                    ,array( $this, 'render_meta_box_content' )
                    ,$post_type
                    ,'advanced'
                    ,'high'
                );
            }
    }

    /**
     * Save the meta when the post is saved.
     *
     * @param int $post_id The ID of the post being saved.
     */
    public function save( $post_id ) {

        /*
         * We need to verify this came from the our screen and with proper authorization,
         * because save_post can be triggered at other times.
         */

        // Check if our nonce is set.
        if ( ! isset( $_POST['myplugin_inner_custom_box_nonce'] ) )
            return $post_id;

        $nonce = $_POST['myplugin_inner_custom_box_nonce'];

        // Verify that the nonce is valid.
        if ( ! wp_verify_nonce( $nonce, 'myplugin_inner_custom_box' ) )
            return $post_id;

        // If this is an autosave, our form has not been submitted,
                //     so we don't want to do anything.
        if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
            return $post_id;

        // Check the user's permissions.
        if ( 'page' == $_POST['post_type'] ) {
            if ( ! current_user_can( 'edit_page', $post_id ) )
                return $post_id;
        } 

        else {
            if ( ! current_user_can( 'edit_post', $post_id ) )
                return $post_id;
        }

        /* OK, its safe for us to save the data now. */

        // Sanitize the user input.
        $mydata = sanitize_text_field( $_POST['myplugin_new_field'] );
                        $mydatatest = sanitize_text_field( $_POST['myplugin_new_check_field'] );

        // Update the meta field.
        update_post_meta( $post_id, '_my_meta_value_key', $mydata );
                        update_post_meta( $post_id, 'test', $mydatatest );
    }


    /**
     * Render Meta Box content.
     *
     * @param WP_Post $post The post object.
     */
    public function render_meta_box_content( $post ) {

        // Add an nonce field so we can check for it later.
        wp_nonce_field( 'myplugin_inner_custom_box', 'myplugin_inner_custom_box_nonce' );

        // Use get_post_meta to retrieve an existing value from the database.
        $value = get_post_meta( $post->ID, '_my_meta_value_key', true );

        // Display the form, using the current value.
        echo '<label for="myplugin_new_field">';
        echo '</label> ';
        echo '<input type="text" id="myplugin_new_field" name="myplugin_new_field"';
                echo ' value="' . esc_attr( $value ) . '" size="25" />';
        _e( 'Yes', 'myplugin_textdomain' );

    }
}

Which is displayed in the back-end like so:

---

It is currently using a text field, however...when I change it to a checkbox input field type (which is what I am after), it won't store this information and continues to display an empty checkbox.

echo '<input type="checkbox" id="myplugin_new_field" name="myplugin_new_field"';

How can I successfully save and store this information, similar to how the text field settings work?

---

Secondly, in my header.php file I have the following lines of code:

<?php echo get_post_meta($post->ID, '_my_meta_value_key', true); ?>
<div class="call-to-action-sticky">
    <p class="orange-button">Call 123 456 789 </p>
</div>

How can I create an if statement so that the above div content will only display depending on if the back-end user has checked the checkbox?