Submit button doesn't work

Most likely a JS script problem. Is there any console error when you click submit?

No visible error. ‘Loading” message stays on for ever. Which JS script I should look for?

Any JS error, what is your site URL with the problem?

Firebug showing 403 forbidden error on admin-ajax.php when ‘Mail it’ button is clicked.

This might help https://lesterchan.net/wordpress/2012/06/05/ajax-not-working-for-wp-email-wp-polls-wp-postratings-or-wp-postviews/ (2nd point)