Much like with plugins, the currently active theme can be deactivated manually if access to the dashboard is temporarily unavailable. To do so, first gain access to the WordPress install’s files using FTP, SFTP, SSH or file manager access through your host’s control panel. Next navigate to wp-content/themes and locate the currently active theme. At this point, all you need to do is either rename that folder to something different or move it to a different directory. Once you’ve done that, attempt to visit the site again and see if the site comes back.
If this works, and the theme was causing the fatal error, WordPress will automatically make the current default theme active.