Perform action/email when a WooCommerce order is placed on-hold

functions.php

<?php
/**
 * Perform action/email when a WooCommerce order is placed on-hold
 */
function wps_woocommerce_order_status_changed( $order_id, $checkout = null ) {
	global $woocommerce;
	$to = 'your-email@yourdomain-name.com';
	$subject = '';
	$body = '';
	$headers = array('Content-Type: text/html; charset=UTF-8');

	$order = new WC_Order( $order_id );
	if($order->status == 'on-hold'){
		$subject = 'Order #' . $order_id . ' has been placed on-hold';

		$body = '<p><strong>WooCommerce Order <a href="' . home_url( '/' ) . 'wp-admin/post.php?post=' . $order_id . '&action=edit" target="_blank">#' . $order_id . '</a> has been placed on-hold' . '</strong></p>';

		$body .= '<p>';
		$body .= $order->get_billing_first_name() . ' ' . $order->get_billing_last_name() . '<br />';
		$body .= $order->get_billing_company() . '<br />';
		$body .= '</p>';

		$body .= '<p>';
		$body .= '<strong>Address:</strong><br />';
		$body .= $order->get_billing_address_1() . '<br />';
		if( !empty( $order->get_billing_address_2() ) ) {
			$body .= $order->get_billing_address_2() . '<br />';
		}
		$body .= $order->get_billing_city() . ' ' . $order->get_billing_state() . ' ' . $order->get_billing_postcode() . '<br />';
		$body .= $order->get_billing_country() . '<br />';
		$body .= '</p>';

		$body .= '<p>';
		$body .= '<strong>Email:</strong><br />';
		$body .= $order->get_billing_email();
		$body .= '</p>';

		$body .= '<p>';
		$body .= '<strong>Phone:</strong><br />';
		$body .= $order->get_billing_phone();
		$body .= '</p>';

		wp_mail( $to, $subject, $body, $headers );
	}
}
add_action('woocommerce_order_status_changed','wps_woocommerce_order_status_changed');