There are several cases for the cause and multiple solutions also:
1 Add these 2 lines in wp-config.php file
if (strpos($_SERVER['HTTP_X_FORWARDED_PROTO'], 'https') !== false)
$_SERVER['HTTPS']='on';
2 Edit these settings in database directly using PhpMyAdmin in table options or wp_options in option_id, option_name (1,siteurl) and (2,home)
3 You can do the same by Editing wp-config.php and adding these 2 lines
define('WP_HOME','https://example.com');
define('WP_SITEURL','https://example.com');
4 Fix your .htaccess file with default one
Check here for default .htaccess for wordpress
