<ul class="product-menu">
              <?php
              $products = new WP_Query(
                $args = array(
                  'post_type' => 'product',
                  'post_status' => 'publish',
                )
              );
              while ( $products->have_posts() ) {
                $products->the_post();
                $post_thumbnail_id = get_post_thumbnail_id();
                $post_thumbnail_url = wp_get_attachment_url( $post_thumbnail_id );
                echo
                '<li>
                  <a href="' . the_permalink() . '">
                    <img src="' . $post_thumbnail_url . '" alt="' . get_the_title() . '">
                  </a>
                </li>';
              }
              /* Restore original Post Data */
              wp_reset_postdata();?>
              </ul>

<ul class="product-menu">
    <?php

    $args = array(
        'post_type' => 'product',
        'post_status' => 'publish',
    )

    $products = new WP_Query($args);

    while ( $products->have_posts() ) {
        $products->the_post();

        echo '<li><a href="' . get_the_permalink() . '">';

        // check if the post has a Post Thumbnail assigned to it.
        if ( has_post_thumbnail() ) {
            the_post_thumbnail();
        }

        echo '</a></li>';
    }
    /* Restore original Post Data */
    wp_reset_postdata();

    ?>
</ul>