I have been using One Page Checkout on this site since March with no difficulties: http://geetac.co.uk
Last week, it suddenly stopped working and now users cannot checkout because when they enter the country in the checkout form, shipping options no longer appear. There is a blank pop up that appears, but nothing else happens.
I haven't touched this site for weeks, so I know it is no change that I have made that has caused OPC to suddenly stop working. I can't see any evidence of hacking, although this is always a possibility of course. But I suspect it is something else.
Please help! It's a live shop site and the owners are very unhappy that the customers can't make their purchases at the moment.