I can’t call my PHP MVC model classes to my site

The function I use to include my model files system/libs/load.php

   public function model($fileName)
    {
        include "app/model/" . $fileName . "_model.php";
        return new $fileName();
    }

this is how i call my model file.

   public function again()
    {
        $models = $this->load->model("index");
    }

I get this error when I open the browser:

currently unable to process this request