Missing featured image in post editor

I must be going crazy …

I wished to add a new post to my website today for the first time in a very long time, and in the WordPress post editor, the option to define a featured image … is missing. Perhaps it disappeared following a WordPress or plugin update.

I use the following code to reference the featured image of a post on my homepage:

src=”<?php $fi = get_post_custom_values(‘featured_image’); echo $fi[0]?>”>

Can anyone please tell me how to add a field on on my post editor so I can once again input the URL featured image for a given post?

Thanks in advance