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My web site has been working fine for several months and all of a sudden last week I get this messag:
Hello. Thank you for loading Zen Cart™.
You are seeing this page for one or more reasons:
This is your first time using Zen Cart™ and you haven't yet completed the normal Installation procedure.
If this is the case for you, you will need to upload the "zc_install" folder using your FTP program, and then run zc_install/index.php via your browser (or reload this page to see a link to it).

Your /includes/configure.php and/or /admin/includes/configure.php file contains invalid path information and/or invalid database-connection information.
If you recently edited your configure.php files for any reason, or maybe moved your site to a different folder or different server, then you'll need to review and update all your settings to the correct values for your server.
See the Online FAQ and Tutorials area on the Zen Cart™ website for assistance.
Additional Details: includes/configure.php file contents invalid. ie: DIR_FS_CATALOG not valid or not set

To begin installation ...
The Installation Documentation can be read by clicking here: Documentation
You will need to upload the "zc_install" folder using your FTP program, and then run zc_install/index.php via your browser (or reload this page to see a link to it).
The Online FAQ and Tutorials area on the Zen Cart™ website will also be of value if you run into difficulties.

I have not made any changes to the configure.php files

Thanks

Exactly as the message says: your /includes/configure.php file no longer contains valid information.

So, whether you changed it or not, it's been changed.

So, you need to find out WHO changed it, HOW it got changed, WHY it was changed, and get it put back to whatever it should be.

Maybe your hosting company did something to the servers, or relocated your site to another server.
Maybe one of your coworkers has been messing around in FTP doing something to the site, and made changes and didn't understand what they were doing.
Maybe you gave someone your password and they've misused it.
Maybe your site is using insecure addon code and has allowed an intruder into it.

It's a good thing you have really good regular site backups of all your PHP files and your database, so you can restore it quickly, right?

My server was changed. I modified the includes/config.php DIR_FS_CATALOG to have the complete path and now I get a 500 Internal Server Error.

If the server was changed, then your configure.php file contents are probably bad, not just in one line. You'll need to update all the settings to reflect the new paths.

For reference, the PROPER way to change servers is here: https://www.zen-cart.com/tutorials/index.php?article=100