This will only occur if either the editor is now permitted for the user group that is viewing that edit page (if it's JCE it does have options for that) OR because there is some JS error on that page caused by some other plugin. The latter is easy to solve. Instead of opening the modal to edit, just right click and open in a new tab. Then open your browser's console (in dev tools) and check for any errors after you reload the edit page.