Fatal Error after editing custom files

I've been renaming the headers on my side boxes and things were working well until I started renaming my Document Categories and EZ Pages headers. I edited these files:
includes/language/english/extra_definitions/document_general.php
includes/language/english/extra_definitions/ez_pages_definitions.php

After renaming the headers, I got this message:
Fatal error: Call to a member function on a non-object in /home/cobrapla/public_html/store/includes/classes/category_tree.php on line 68

The main page loaded fine until I clicked on a link. After that every page had the above message where the Document and EZ Page side boxes should be.

I didn't touch the category_tree.php file prior to this occurring. I checked that particular file, and I didn't see any codes on line 68. Anyone have any thoughts on how to fix this?

What happens if you delete those added files?
Undoing the things you recently did should help you get back to the working state prior to when it broke.

I deleted those files (document_general.php and ez_pages_definition.php) as you've suggested, but I still get the same message. I also reloaded the original files that I have as a back up, and I still get that same message.

I've read other threads about fatal errors, but I'm not seeing anyone else with this particular issue. I also checked myPHPAdmin, but I didn't see category_tree.php in there. I'm not even sure if it should be there, but I thought I'd at least look around a bit.

Sounds like you may have uploaded extra files to the wrong places or maybe something's gotten damaged on the server.

This article discusses the best way to identify differences:
http://www.zen-cart.com/wiki/index.php/Troubleshoot_-_Diagnosing_Obscure_Issues

Thanks for the info! It'll be a while before I can run it the comparison. I'll keep you posted once I complete it.