Featured Image missing since upgrade

The featured image box has disappeared recently from my posts – I *think* since I ran the most recent WP upgrade. I’ve checked in the ‘Screen Options’ tab at top of page and its not there to display. Have already tried disabling all plugins to see if there’s any conflict, but its missing even with no plugins enabled.
Strangely enough, the featured image thumbnails are still present on my index etc. where they should show up, for all posts up to now, but of course my new posts don’t show a featured image thumbnail since there’s nowhere for me to list it!