osCommerce? You can get help on their discussion forum (http://forums.oscommerce.com/community). If you wiped out your application_top.php file, and you don't have a backup (either your own, or LP's nightly backup -- $$) to retrieve a fresh copy from, you can go to osCommerce and download the appropriate osC version installation package, unzip it, and extract the missing files. Depending on how customized your store was, you might have to start with all fresh files and apply customizations again. In that case, use the latest production version, 2.3.1. You should be able to preserve your database and product pictures, at least within the same version of osC. By the way, there are two different application_top.php files -- in includes/ and admin/includes/. Don't confuse the two.

Then you will learn to periodically download a full backup of your site files to your PC, as well as a backup of your database.