Adding thumbnail to search results & changing content.php

Use the_post_thumbnail. For example:

<?php if ( has_post_thumbnail()) : ?>
   <a href="https://wordpress.stackexchange.com/questions/250403/<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>" >
   <?php the_post_thumbnail(); ?>
   </a>
<?php endif; ?>

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)