This is a child theme error, delete the child theme via ftp. You can copy it to your desktop before deleting. Another option: If your site was working fine before then ask your host to restore 24 hours back to get back online.
To find issues with your child theme, download the official child theme from member area and cross check. Make sure all directories and files are present as in the official child theme.
If your your child theme has design/CSS changes only then you can use parent theme and paste the CSS in custom.css or in Customizer -> Additional CSS even without using the child theme.