Blank page when executing sql command [closed]

When I run the index.php page, there is a form that goes to the login.php page

When the login page is executed it is completely blank, it does not execute anything.

The code is as follows:

<?php
include_once("../../conf.php");

$mysqli = mysqli_connect(HOST,user,PASS,DB) or die (mysqli_error());

$login = $_POST['login'];
$senha = $_POST['senha'];

if ($prep = $mysqli->prepare("SELECT * FROM Admin WHERE login=? AND password=?")) {
    $prep->bind_param('ss',$login,$senha);
    $prep->execute();

    if ($prep->error) {
        echo "erro";
    } else {
        echo "ok";
    }
} else {
    echo "erro 2";
}

The directory where conf.php is, there is a simple connection to the database using mysqli_connect.

What can happen? Not an error message or anything ..