codeigniter3 file uploading not working

I have facing a problem on codeigniter3 file upload. getting following response 

"is currently unable to handle this request.

HTTP ERROR 500"


Not getting any errors.  but validation working fine.
anybody can  suggest me a solution fr this. Thank you.
following are my code
$this->load->helper('url');

$this->load->library('pagination');

$this->load->model('Video_model');



if($this->input->post('submit')){

$upload_path = './upload/';

           $config['upload_path'] = $upload_path;

           //allowed file types. * means all types

           $config['allowed_types'] = 'wmv|mp4|avi|mov';

           //allowed max file size. 0 means unlimited file size

           $config['max_size'] = '50000';

           //max file name size

           $config['max_filename'] = '255';

           //whether file name should be encrypted or not

           $config['encrypt_name'] = true;

           //store video info once uploaded

           $video_data = array();

           //check for errors

           $is_file_error = FALSE;

           if (!$_FILES) {

               $is_file_error = TRUE;

               $this->handle_error('Select a video file.');

           }



         

if (!$is_file_error) {



               //load the preferences

               $this->load->library('upload', $config);

             

               //check file successfully uploaded. 'video_name' is the name of the input

               if (!$this->upload->do_upload('userfile')) {

                   //if file upload failed then catch the errors

                   $this->handle_error($this->upload->display_errors());

                   $is_file_error = TRUE;

               } else {

                   //store the video file info

                   $video_data = $this->upload->data();

                   

                  

                  $this->save_video($video_data['file_name']);

               }
       }