What are you typing in the src="https://wordpress.stackexchange.com/questions/82463/xxxxxx"? Do you have the absolute path or relative path to your style?
If your not loading the CSS from your functions.php with wp_enqueue_style you should load it like this in your header to always get the correct URL to the file: <link rel="stylesheet" type="text/css" href="https://wordpress.stackexchange.com/questions/82463/<?php echo get_stylesheet_uri(); ?>">
Update:
If you (for some reason unknown) want to keep your current file structure with the CSS outside your theme folder. Use this code instead: <link href="https://wordpress.stackexchange.com/questions/82463/<?php echo home_url("https://wordpress.stackexchange.com/" ); ?>css/style.css" rel="stylesheet" type="text/css" />