This morning, thumbnails showed for the 50+ posts on my site, but all the posts had the blank boxes with the name of the photo that should be there. I went through them one by one (500 pictures) and used the Replace button, which restored the original pictures. A terrible chore, but it worked.
I've now, though, accidentally found what the issue was. The URLs for the photos had suddenly lost the / (slash) that should have been in wordpress(/)wpcontent.
I'd like to know how this disappeared (upgraded to 3.9?) and whether there was a simpler way to go back and restore all the missing slashes to repair the URLs.
Hate this sort of confidence-knocking thing. It happened to me even worse on the last upgrade. That time, I had to actually re-upload every photo.