HTTP 500 error after installing plugin

I have WordPress and BBPress installed, and I added the Cartpauj PM plugin http://wordpress.org/plugins/cartpauj-pm/ to have a private messaging system on the site. The plugin installed successfully, but I found an unwanted reply form on the botton of the private messages page on the website. So I deleted this from one of the PHP files of the plugin:

//SHOW THE REPLY FORM
      $threadOut .= "</table>
      <p><strong>".__("Add Reply", "cartpaujpm").":</strong></p>
      <form name='message' action='".$this->actionURL."checkmessage' method='post'>".
      $this->get_form_buttons()."
      <textarea name='message_content'></textarea>
      <input type='hidden' name='message_to' value='".$this->convertToUser($to)."' />
      <input type='hidden' name='message_title' value='".$re.$message_title."' />
      <input type='hidden' name='message_from' value='".$user_ID."' />
      <input type='hidden' name='message_date' value='".current_time('mysql', $gmt = 1)."' />
      <input type='hidden' name='parent_id' value='".$pID."' />
      <input type='submit' onClick='this.disabled=true;this.form.submit();' value='".__("Send Message", "cartpaujpm")."' />
      </form>";

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This is the PHP file that it was deleted from:
http://we.tl/45hrmdQ8uX

Now the server reports HTTP 500 error for the whole website, even the admin dashboard is unaccessible at the subdirectory /wp-admin/.I found this odd, because this private messages plugin only adds a page and controls that, a separate page, no embedded links on the site, yet now the whole site is gone, even the dashboard. How it is possible? Or maybe the site was hacked while I was managing it?