Problems when retrieving array with MySQL info

you are not returning the data of the query outside of your function:

try this way:

public function consulta(){
    $sentencia = 'SELECT * FROM Usuarios';
    $result = mysqli_query($this->con, $sentencia);
    $fila = mysqli_fetch_assoc($result);
    return $fila;

}

and in the function of the query, it is a single record that you take out of the database, thus proves the validation:

$datos = $db->consulta();
$pass_bd = $datos['contrasena'];
$user_bd = $datos['usuario'];
if (($user_bd = $user) AND (password_verify($pass_bd,$hash))) {
    session_start();
    $_SESSION['logueado']=TRUE;
    header('Location: control.php');
}
else{
    header('Location: login.php');
    echo "Error";
}

because a foreach is not required for what you are trying to do. By the way, it filters the data that it searches for users for auitentication, something like this:

public function consulta($X){
    $sentencia = 'SELECT * FROM Usuarios WHERE usuario = $X';
    $result = mysqli_query($this->con, $sentencia);
    $fila = mysqli_fetch_assoc($result);
    return $fila;

}

and queries like this:

$datos = $db->consulta($_POST['usuario']);

The failure is that you are not returning the data from your sentence as you have already mentioned, even so, I have modified the function of your query a bit so that it is more useful, so you can work with different sentences and not a single one .

On the other hand, your password_verify is not going to work for you, you're having both parameters and the first one should not be wasted.

Your code:

password_verify($pass_bd,$hash)

Remember, $pass_bd must be wasted in your Database for security and to verify it you must do the following:

//Contraseña usuario.
$pass = $_POST['contrasena'];
//Verificamos contraseña.
password_verify($pass,$pass_bd)

Possible example:

Database

require_once('conf_db.php');

class BaseDatos{

    protected $con;
    protected $db;
    protected $registro;

    public function conectar() {
        $this->con = mysqli_connect(HOST, USER, PASS, DBNAME);
        if ($this->con == '0') DIE("Lo sentimos, no se ha podido conectar con MySQL: ".mysqli_error());
        //$this->db = mysqli_select_db(DBNAME, $this->con);
        if (DBNAME == '0') DIE("Lo sentimos, no se ha podido conectar con la base datos: ".DBNAME);
        return true;
    }

    public function desconectar() {
        if ($this->conectar->con) {
            mysqli_close($this->$con);
        }
    }

    //Pasamos nuestra consulta directamente.
    public function consultaRetorno($sql){            
        $result = mysqli_query($this->con, $sql);    
        return $result;                
    }
}

We check the user

<?php 

    include('../include/conexion.php');
    $db = new BaseDatos();
    $db->conectar();        

    if($db->conectar()){
        //Datos recogidos de login.php
        $user = $_POST['usuario'];
        $pass = $_POST['contrasena'];
        //Para una mayor seguridad pasamos la contraseña a un hash
        $hash = password_hash($_POST["contrasena"], PASSWORD_DEFAULT);

        //Creamos sentencia
        $sql = "SELECT * FROM Usuarios WHERE usuario = '$user'";
        //Pasamos sentencia a nuestra función.
        $result = $db->consultaRetorno($sql);
        //Retornamos datos
        $registro = mysqli_fetch_assoc($result);

        if ($registro['usuario'] == $user) {

            if (password_verify($pass,$registro['contrasena'])) {
                session_start();
                $_SESSION['logueado']=TRUE;
                header('Location: control.php');
            } else {
                echo "Error contraseña";
            }

        } else {
            echo "Error usuario";
        }            
    }
?>

  

Note: I advise you for security use mysqli prepare statements or PDO. I also advise you to read well    How to avoid SQL injection in PHP ?