1. Goto
classes\controller\FrontController.php
2. Find the function
public function init()
3. Paste the below mentioned code after
parent::init();
the code
if (!$this->context->customer->isLogged() && $this->php_self != 'authentication' && $this->php_self != 'password')
Tools::redirect('index.php?controller=authentication?back=my-account');
Please keep in mind that if you use this modification on your prestashop 1.5, any website that hot linking to your online store they would become useless / not working. So, you may want to double check for this.