Open console ( right click -> inspect – Console ) and read what is the error, you probably didn’t enqueue jQuery
You should enqueue scripts from functions.php ( best practice )
function theme_scripts() {
wp_enqueue_script( 'jquery' );
wp_enqueue_script( 'script', get_template_directory_uri() . '/script.js', array(), false, true );
}
add_action( 'wp_enqueue_scripts', 'theme_scripts' );