Sidebar section to edit Featured Image has disappeared from the sidebar

Hi, When creating a new post the section to add or change a featured image has disappeared from the right sidebar. The featured image still appears on existing posts. The Media Library is still there. But when editing a post, the right sidebar does not show the section that it used to show for adding/changing featured images. It only shows Summary, Yoast, Revisions, Categories, Tags etc... but not Featured Image. Where has it gone? How can I get it back? Screenshot below. All help gratefully received, thank you.