Regenerate Thumbnails killed making thumbnails

I used the plugin “Regenerate Thumbnails” on some images on our website and found that it deleted all the thumbnails and then left only the original size photo. It didn’t create thumbnails at all. But that’s not the real problem. The real problem is after I uninstalled the plugin now when I upload a new image into the Media folder, all that ends up there is the original size photo. WordPress is no longer generating thumbnails.

What do I need to do to fix this.