Accidentally disabled "Categories" (new to PHP code)

So I have been gradually learning how PHP works, but I am still new to it, and I think I have inadvertently killed the Category URLs.

The category names continue to show up via the Category widget on the right panel, and each displays the correct amount of articles in each respective category and subcategory, but following any links or entering the URL to access the categories themselves prompts an HTTP Error 500 notification.

I am sure this is easy to remedy, as it seems to have been a coding error on my part, but being so new to PHP, I am struggling to locate the file that needs to be repaired.

For your reference, the page in question is: Evolve with EVO.

Any input would be greatly appreciated. Many thanks in advance, you people rock! =)