Get wp_get_attachment_url outside of loop

if the result you’re looking for is a printout of the URL, like in your example, then this should work:

$page_id = get_queried_object_id();
if ( has_post_thumbnail( $page_id ) ) :
    $image_array = wp_get_attachment_image_src( get_post_thumbnail_id( $page_id ), 'optional-size' );
    $image = $image_array[0];
else :
    $image = get_template_directory_uri() . '/images/default-background.jpg';
endif;
echo $image;