I had thumbnails set up and working perfectly on my site. However, I wanted to change the size and did so through the functions.php file (although I see this is now an option in Settings -> Media). I then used the plugin Regen. Thumbnails. It went through all the images and seemed to be successful. However, this has now broken all of my thumbnails. I can see it did not create new images in the uploads folder (just the original 150x150px thumbnails). All the links through to thumbnails now just link through to the full size of the image. If I upload a new image, a thumbnail will not be generated.
I've restored the thumbnail settings back to the original settings, no luck (also tried regenerating them at the original size even though these are still present in the uploads folder).