URGENT - Add to cart not working!!

Hope this is the right place for this post - I need urgent help as all of a sudden, and for no apparent reason, the 'Add to cart' button has stopped working on all three of my Zen sites.

The only changes I have made recently (ie last night) are:
Deleting text through Define Pages
Changing the payment type from 'Authenticate' to 'Immediate Charge' in my Protx Direct module.

The second of these seems to have brought up a CURL error, so that has now been changed back to 'Authenticate'.

Visit either of these, and try going into a product and adding to your cart, and you just get an 'Internet Explorer cannot display the web page' message.
http://www.diyfavourboxes.co.uk
http://www.diy-wedding-favours.co.uk
http://www.one-eyed-cat.co.uk

I am at a loss...

Any help would be HUGELY appreciated!

Thanks

Sara
PS the 'Buy now' button seems to work fine, it's just the 'Add to cart' that's causing problems.

Contact your host, see if they made any change last nite.