Hi,
I first think that it looks like a hidden PHP Fatal error in your email.
Then I see that you send us an email with accesses to your website ; so I take a little look.
I forced temporally the "error reporting" to "maximum" in your Joomla global configuration and I was able to see the PHP error message
Parent/Guardian Name : ---------
Telephone : -------------
How did you hear about this holiday's labs? : -------------
Child's First Name : -------------
Child's Surname : -------------
Date of Birth : -------------
School Year : -------------
School : -------------
Anything else that you might want to add : -------------
Warning: strtoupper() expects parameter 1 to be string, object given in /YYYYYY/public_html/XXXXXXXX/administrator/components/com_hikashop/classes/field.php on line 1382
Catchable fatal error: Object of class stdClass could not be converted to string in /YYYYYY/public_html/XXXXXXXX/media/com_hikashop/mail/order_status_notification.preload.modified.php on line 378
I don't know if you have customized your preload email, but if you did not modify it, I will suggest you to remove the override for that part of the email.
Regards,