Can you access your site via FTP or through your host control panels file manager?
If you can, check to see if there is any code within wp-comments-post.php in the root of your site.
If it is empty, paste this inside:
<?php
/**
* Handles Comment Post to WordPress and prevents duplicate comment posting.
*
* @package WordPress
*/
if ( 'POST' != $_SERVER['REQUEST_METHOD'] ) {
header('Allow: POST');
header('HTTP/1.1 405 Method Not Allowed');
header('Content-Type: text/plain');
exit;
}
/** Sets up the WordPress Environment. */
require( dirname(__FILE__) . '/wp-load.php' );
nocache_headers();
$comment = wp_handle_comment_submission( wp_unslash( $_POST ) );
if ( is_wp_error( $comment ) ) {
$data = $comment->get_error_data();
if ( ! empty( $data ) ) {
wp_die( $comment->get_error_message(), $data );
} else {
exit;
}
}
$user = wp_get_current_user();
/**
* Perform other actions when comment cookies are set.
*
* @since 3.4.0
*
* @param WP_Comment $comment Comment object.
* @param WP_User $user User object. The user may not exist.
*/
do_action( 'set_comment_cookies', $comment, $user );
$location = empty( $_POST['redirect_to'] ) ? get_comment_link( $comment ) : $_POST['redirect_to'] . '#comment-' . $comment->comment_ID;
/**
* Filter the location URI to send the commenter after posting.
*
* @since 2.0.5
*
* @param string $location The 'redirect_to' URI sent via $_POST.
* @param WP_Comment $comment Comment object.
*/
$location = apply_filters( 'comment_post_redirect', $location, $comment );
wp_safe_redirect( $location );
exit;
?>