wordpress get_the_post_thumbnail is not working

Hello friends i used get_the_post_thumbnail() this function like.

get_the_post_thumbnail(get_the_ID(), "thumbnail");

This is working properly and return the thumbnail images.

But i write like this is not working.

get_the_post_thumbnail(55, "thumbnail");
or
get_the_post_thumbnail('55', "thumbnail");

This is not return any thing please help. I’m using the $wpdb->get_results();

thank you.