[Plugin: Cart66] Ajax Error?

I have a site at https://www.artworkspa.com that was working a couple weeks ago (The last order was 8/29/12). I got a call yesterday from someone saying they couldn’t add anything to the cart anymore. Now it when you click on add to cart it gives a popup that says readystate:4 status:200. It doesn’t proceed to cart or anything.

Any ideas would be appreciated.