Code php wrongly and wp page stop working

You do not put shortcodes in the functions.php file. You put them where you want the shortcode to run. Most likely a page or post.

Don’t put that code into functions.php. It should go on the theme file where you want it to display, maybe page.php or single.php. If you put it after the_content() on page.php for instance, it would display on every page of your site after the content.

If you want to use the shortcode in a specific page or post, you’d go into the control panel and edit the page, then insert just this where you want it to appear:

[yith_woocommerce_ajax_search]

… and once you FTP in, find your theme in wp-content/themes, download the functions.php file – make a copy as a backup – then edit it and re-upload into your theme.