All store pages suddenly blank, admin views ok

My store, running on 1.3.8a is suddenly showing blank pages.

The link to the store is http://www.artellaland.com/shop

Any file is showing up blank, except that admin is working correctly.

The last changes that were made were just to edit a couple of products in the Catalog (to change the Manufacturer) - so no php edits made, and nothing was uploaded via FTP. Everythign was working fine at the time of these changes.

There are no Debug log files in the Cache folder.

No server/hosting changes or other FTP changes have been made. I know I need to upgrade, but I can't imagine why this would suddenly occur.

Any suggestions? Unfortunately this occured during the last 2 days of our bigget sale of the year, so I'm needing a fix.

Updated info: I ran the Php debugger and found this error:

Fatal error: Call to undefined function zen_update_products_price_sorter() in /home/artellal/public_html/shop/includes/functions/specials.php on line **70

The line of code in question is:
****** zen_update_products_price_sorter($specials->fields['products_id']);****************

Any idea how to fix?********

When I do a page source I see a " 1 "...

Maybe a hack?

FYI - problem solved. zilog357's solution here worked for me:
http://www.zen-cart.com/showthread.php?143956-Zen-Cart-pages-blank-Admin-is-fine&highlight=Call+undefined+function+zen_update_products_price_sorter%28%29