I tried to make changes in child theme bistro.When I made changes in function.php.Site is nort cominup and not even wordpress wp-admin area.The site coming blank.Site name is www.assurancegroup.co.uk
You should have made your changes in functions.php using FTP or cPanel. I assume you tried to make them from the Editor. You need to log in from FTP and delete the Bistro folder.
If you cant copnnect through ftp then go to your hosting control panel.Then go to your website folder-> wp themes-> and delete bistro.It happened with me to but i recover it.