Checkout not completing

This doesn't seem to be a browser issue as it is occurring across multiple browsers. I haven't made any changes to my site recently, but suddenly, the checkout process is not completing. The products get added to the cart, but when you click the "next" button, it returns to the same page. It looks like some others have had the issue in IE9, but I have tried on other browsers and the problem repeats.
The site is at www.georgiaswimrig.com

Any help would be greatly appreciated. I can provide access to the backend if necessary.