A few things of note here:

a. Using PayPal Payments Standard makes you reliant on a number of things that can easily go wrong if the server and other factors aren't set up properly. One of the symptoms of that is orders not showing up in the store because the final activation doesn't happen due to those misconfigurations.
The simplest fix for that is to switch over to using PayPal Express Checkout ... (which has the same fee structure as PayPal Standard) and provides a better experience to the customer anyway.
Switching is simply a matter of clicking Remove on PayPal Standard and clicking Install on Express Checkout and adding your PayPal credentials (API signature and username and password). There's a link in the help text next to the Install button explaining it, with a convenient link to obtaining credentials too.

b. The sudden problem of broken transactions since beginning of December is probably connected to the POODLE SSL problem internet-wide. To fix your store, simply follow the instructions here to make a simple fix to a couple files: http://www.zen-cart.com/showthread.p...yment-security

c. It's probably worth taking a look at what errors are occurring on the site, and fixing those. You can find the myDebug-xxxx.log files in the /logs/ folder, using FTP. Explained more here: http://www.zen-cart.com/content.php?124-blank-page