Show Featured Image in else statement

The featured image-related functions do not offer any way to define a default/fallback image. So, you’ll have to define one yourself, e.g.

<?php $featured_image = ( '' != get_the_post_thumbnail() ? get_the_post_thumbnail() : get_template_directory_uri() . '/images/img_not_available.png' ); ?>

<a href="https://wordpress.stackexchange.com/questions/118789/<?php the_permalink(); ?>"><?php echo $featured_image; ?></a>

Edit

Three changes:

1. More explicit conditional. get_the_post_thumbnail() returns an empty string rather than false if no image is found.
2. Updated the default image with yours. Note: use get_template_directory_uri() and not get_stylesheet_directory_uri().
3. Cleaned up interspersed PHP/HTML, to help eliminate syntax errors

Note also: example code assumes a context of inside the loop. Modify accordingly if it is used outside the loop.