Checkout infinite loop when clicking "next"

Joomla (we're on 1.5.18) seems to have a spontaneous glitch on our site, whiskerworks.com. Customers are not able to check out anymore. When they login or otherwise identify themselves and select their shipping and payment methods, then click next ... the site idles for a while "thinking" then the user is returned either to the same page they were on before, complete with all of their information, cart data, shipping and payment information still as it was before, or the customer will be sent to a page that simply says "Thank you for your order." At no point is the user given any forms or fields to fill in a credit card number, go to PayPal or anything else of the sort.

Mind you, the checkout process worked fine until Saturday. We still receive e-mail notification when orders are placed, just no one can pay for their orders so we're in quite a bind. Any help is appreciated.

Hi,

What is the version of HikaShop ?
Have you changed anything on your website since Saturday ?

Thanks to enable the Joomla debug mode, and set the error reporting to maximum to see if an error is returned.