How to check an encrypted password during Login?

Important this answer was given to the situation posed in the question.

Do not use crypt and Salt fixed to save passwords, and in PHP use the function password_hash and password_verify for passwords.

original answer:

Hash again the password that the user entered, and see if it hits the DB:

// Pego a senha do POST (adapte pro seu código)
$password = $_POST['password'];

// Aqui estamos fazendo o mesmo que você usou para encriptar. Mas dá pra melhorar isso.
$cost = '11';
$salt = 'Cf1f11ePArKlBJomM0F6aJ';
$password_hash = crypt($password, '$2a$' . $cost . '$' . $salt . '$');

// Agora comparamos o HASH da senha digitada com o HASH da senha salva
// porém, deste jeito, você está vulnerável a SQL Injection.
$mysqli->query(
   "SELECT * FROM user WHERE username = '".$username."' AND password = '".$password_hash."'"
);

See how the above code looks using bind_param, to avoid SQL Injection :

// Pego a senha do POST (adapte pro seu código)
$password = $_POST['password'];

// Inicializamos o id do usuário com 0 para usar no fetch()
$idUsuario = 0;

// Aqui estamos fazendo o mesmo que você usou para encriptar. Mas dá pra melhorar isso.
$cost = '11';
$salt = 'Cf1f11ePArKlBJomM0F6aJ';
$password_hash = crypt($password, '$2a$' . $cost . '$' . $salt . '$');

// Agora comparamos o HASH da senha digitada com o HASH da senha salva:
$query = 'SELECT id FROM user WHERE username = ? AND password = ?';
$stmt = $mysqli->prepare( $query );
$stmt->bind_param("ss", $username, $password_hash );
$stmt->execute();

// Aqui pegamos o resultado.
// se for mais de um campo, pode ser bind_result( $idusuario, $username, $email...
$stmt->bind_result( $idUsuario );
$stmt->fetch();

if ( $idUsuario ) {
   echo 'Logado';
} else {
   echo 'Usuario e/ou senha invalidos';
}

Answer updated with question edit:

Follows refactoring using the principles indicated:

<?php
   $page = 'Login';
   session_start();
   include 'header.php';
   $user_error = '';
   $pass_error = '';

   if(isset($_SESSION['username'])){
      header('location: control-painel.php'); 
   } else {

      if( isset($_POST['login'])) {
         $username = $_POST['username'];
         $password = $_POST['password'];
         $cost = '11';
         $salt = 'Cf1f11ePArKlBJomM0F6aJ';
         $password_hash = crypt($password, '$2a$' . $cost . '$' . $salt . '$');
         $id = 0; 

         if(empty($username)){
            $user_error = 'Please insert a username';
         }
         if(empty($password)){
            $pass_error = 'Please insert a password';
         }

         if( empty( $user_error ) && empty( $pass_error ) ) {
            $mysqli = new mysqli//ABRIR SUA CONEXAO AQUI CASO NAO ESTEJA NO HEADER
            $stmt = $mysqli->prepare( 'SELECT id FROM user WHERE username = ? AND password = ?' );
            $stmt->bind_param( 'ss', $username, $password_hash );
            $stmt->execute();
            $stmt->bind_result( $id );
            $stmt->fetch();

            if($id){
               $_SESSION['username'] = $username;
               header('location: control-painel.php');
               die();
            } else {
               $user_error = 'User or Password invalid';
            }
         }
      }
   }
?>

<div class="message">
   <br><br>
   <?php echo $user_error; ?><br><br>
   <?php echo $pass_error; ?><br><br>
   <br><br>
</div>
<div id="form" class="bradius">
   <div class="content">
      <form method="post">
         <label>Username: </label>
         <input type="text" name="username" class="text bradius">
         <label>Password: </label>
         <input type="password" name="password" class="text bradius">
         <input type="submit" class="submitbutton bradius" name="login" value="Login">
      </form>
   </div>
</div>

<?php
   include "footer.php";
?>