Error 500 - Internal server error

[deitech]

ZenCart ver.: 1.3.7.1
Add-ons: Admin Activity Log Plug-in
Server type: Linux 2.6.28.8
PHP ver.: 5.2.11
MYSQL ver.: 4.0.27

When I try and ANY page in the ZenCart installation, I am met with "Error 500 - Internal server error" and a generic message saying "An internal server error has occured! Please try again later."
The Admin area functions properly and all the products list correctly when viewed by the admin, but once I back out of the admin panel, nothing works!

I'm in a bind here as I'm taking over for a guy who left the owner high and dry (and poorly documented). By all accounts it seems the cart randomly broke one day about 2 months ago and hasn't functioned since. No previous attempts have been made to correct the issue.

I've searched the forums and the only similar issue has the poster giving up. I have attempted to switch the template from the custom design back to default and had no success.

the site is www. ny-trex . com
Clicking the "products" link will display the error (linking to an individual product will display the same error)

All help is appreciated THANK YOU!!!

[mprough]

500 is a server error....

Check the htaccess files for changes. Hostijng company may have upgraded the PHP with a CGI as well.. Which would also be in an htaccess in the root.

~Melanie