All Featured Images Gone/Unassigned

Hi there, all the featured images on my site had not been showing for a while. I had not had time to look into it much, and I assumed it was because of of a theme problem in displaying them, that it would get resolved with an update or something. But it hasn’t… I’ve just looked into it today, and it appears all featured images have been unassigned from all posts.
I haven’t changed the site in a while, and this happened with updates and changes over time.
Is this a problem you’ve ever seen happening? Is there a way to recover these featured images?
Strangely enough, when I go to the edit page of a post, the ‘featured image’ box is empty which I guess makes ssens if featured images have been unassigned. BUT, if I reselect a featured image, the assigned featured image doesn’t appear in the ‘featured image’ box. There used to be a thumbnail of the featured image displayed here once you had selected one. I’m just wondering whether the two issues might be related.
Can you help?
Tx much in advance