Site Down with Server Error

My site vapesales.com is throwing a "HTTP Error 500 (Internal Server Error): An unexpected condition was encountered while the server was attempting to fulfill the request."

The debug file shows "[09-Aug-2012 13:57:57] PHP Fatal error: require() [<a href='function.require'>function.require</a>]: Failed opening required 'includes/templates/template_default/common/html_header.php' (include_path='.:/usr/lib64/php:/usr/lib/php:/usr/share/pear') in /home2/vapesale/public_html/index.php on line 43"

I am php challenged, not very bright, and impatient. Ergo, any help would be greatly appreciated.

lafaverp:

Failed opening required 'includes/templates/template_default/common/html_header.php' ...
The message is telling you that when the system was looking for a required file, it wasn't found on the server.
The file it mentions does indeed come with original Zen Cart files, so either someone has deleted it from your server, or attempted to alter it and failed, or attempted to upload a replacement and the upload failed while at the same time deleting the original.

Assuming you have an established practice of making backups regularly, simply locate that file from your last good full backup and upload it back to your server.

Then figure out who/how it got deleted in the first place.