Can't get my shopping cart sidebox to show up.

I've turned it on in the sideboxes, but it is not showing up on the page.

Sideboxes/shopping_cart.php, column_left.php and tpl_main_page.php seem to match up with the default files so I don't think the problem is any of those files.

Which other files should I be looking in to put my shopping cart sidebox back? When I first started customizing this site, I was doing it through the php files rather than the admin end (didn't realize that all the layout box controls were right in the admin!) so there is a possibility that I may have deleted something somewhere that has to do with the side boxes. I just don't remember where!