Symfony \ Component \ Debug \ Exception \ FatalThrowableError (E_ERROR) . Class 'App\Http\Controllers\DB' Not Found.

It's may be because of use DB;  missing in your controller.

So add controller class use in your controller. So the top of the controller will look like as follows.

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;

use Illuminate\Database\Eloquent\Model;

use App\User;

use DB;