500 internal error message.. stumped!

using 1.3.9
No upgrades
I have installed a template on it and it was working fine. Another designer was in over the weekend and downloaded the tpl_main_page.php file out of my template but said she did not edit it, and next thing she knew the site was down with this error. I'm trying to figure out what she may have did and how to get it up. When i change the template selection to the normal one the site still does not come up correctly.

I installed the site via FTP and it was working for several months since last october till this weekend.

My guess is it is some sort of php coding problem, however I reloaded all my backup files of my template that i used including the tpl_main_page.php

What other things can i check to get this up?

The page with the error is my main index page here: http://www.northshorediamonds.com/catalog

Thanks for any help... very frustrating.

nevermind i solved it... 2 folders were missing. Does anyone know how they may have been deleted if I didn't touch anything nad my other designer didn't as well?

They couldn't have unless your site has been hacked, or someone other than you and your designer has access to FTP or your hosting control panel on your site.