The site is on a VPS, which we manage and we haven't touched it. :(
All transactions on the site have stopped working. My client sent me the following screenshot, which I hope might help you track down the issue. This was with a Visa Debit.
I tried my own personal Mastercard and it didn't produce this massive error script (as per the attached screenshot), but did take me to the checkout_failure.php page. My card is good so there is definitely something wrong.
Hope you can help. Thank you.