Oscommerce problem - Need a solution today

NEED A SOLUTION TO THIS ERROR IMMEDIATELY Fatal error: main() [<a href='[login to view URL]'>function.main</a>]: The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "navigationHistory" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/halfpric/public_html/store/includes/[login to view URL] on line 345 osCommerce 2.2-MS2 PHP Version 5.2.5 Server Host: [login to view URL] ([login to view URL]) Database Host: localhost ([login to view URL]) Server OS: Linux [login to view URL] Database: MySQL 4.1.22-standard Server Date: 20/12/2008 15:24:06 Datebase Date: 20/12/2008 15:24:06 Server Up Time: 15:24:06 up 63 days, 23:33, 2 users, load average: 0.91, 0.93, 1.11 FPRIVATE "TYPE=PICT;ALT=" HTTP Server: Apache/2.0.63 (Unix) mod_ssl/2.0.63 OpenSSL/0.9.7a mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 PHP Version: 5.2.5 (Zend: 2.2.0)