Theme.PHP error

My site has stopped functioning and I’m getting the below error message. It appears to relate to the theme.php file. Any idea what the first step is to resolving this?
Thanks.

Warning: require_once(/home/thefoodw/domains/thefoodwife.co.uk/public_html/wp-admin/includes/class-custom-image-header.php): failed to open stream: No such file or directory in /home/thefoodw/domains/thefoodwife.co.uk/public_html/wp-includes/theme.php on line 2635

Fatal error: require_once(): Failed opening required ‘/home/thefoodw/domains/thefoodwife.co.uk/public_html/wp-admin/includes/class-custom-image-header.php’ (include_path=’.:/usr/share/php’) in /home/thefoodw/domains/thefoodwife.co.uk/public_html/wp-includes/theme.php on line 2635
There has been a critical error on your website. Please check your site admin email inbox for instructions.