500 Internal Server Error (has been working fine for a year)

This zencart store has been functioning fine for about a year. The owner had been away and hadn't checked her site for a few weeks. When she went to her zencart store, she got a 500 Internal Server Error:

The server encountered an unexpected condition which prevented it from fulfilling the request.
The script had an error or it did not produce any output. If there was an error, you should be able to see it in the error log.

Nothing has changed - what could be causing her site to give an error message all of a sudden?

Here is a link to her page:

http://abigailchapin.com/catalog/index.php?main_page=

I know her hosting co. may be a little wacky - she has ipoweb and they broke her cart last year for some time when they changed servers - I am not sure if it has to do with her host again or not - though last time her issue had different symptoms.

Thanks.

To tell more, you should look at the error log file, since this would be the only option to really know what is happening!
It might be that they have changes some server settings and maybe disallowed some .htaccess commands? Thus resulting in an internal server error??

You could also try running a simply php script and see if it works. Try both putting on domain root and in /catalog/!

Code:

<?php phpinfo(); ?>

It could also be a problem with corrupted files on the site, or even a hack attempt.
What version of Zen Cart is on the site?

You might read up on these:
http://www.zen-cart.com/wiki/index.php/Recovering_From_Hacks
http://www.zen-cart.com/wiki/index.php/Troubleshoot_-_Diagnosing_Obscure_Issues