HTTP 500 ERROR

Hi all,
I have a opencart 2.0.1.1 , I have installed for half month and no problem found. But today, when I click a product or category, it appeared HTTP 500 ERROR message. And I followed some websites' shared instructions, add ini_set('display_errors', 1);, ini_set('log_errors', 1);, error_reporting(E_ALL); to index.php. Then appear a message "Fatal error: Class 'Controllerproductcategory' not found in /var/www/html/system/modification/system/engine/action.php on line 62".

Controllerproductcategory seems is refer to category.php:
<?php
class ControllerProductCategory extends Controller {
public function index() {
$this->load->language('product/category');
$this->load->model('catalog/category');
$this->load->model('catalog/product');
$this->load->model('tool/image');
............

action.php on line 62 as below:

$controller = new $class($registry);

How can I fix this problem? (I checked that go to contact us, about us, admin page etc are no problem, just for category and product)

What happens when you undo all the changes you've made?

You may have controller\catalog\category.php file missing from the folder