Delete json to generate the one.
3. If you have image tags to manage your images under Image Manager. You have to copy file tagfolder.js from "json-bak" , and change its path to "json"
The problem could be happened when the upload process is stuck, it was showing the uploaded imaged but not saved correctly. That's why it's not showing again in Image manager.
If to upload them again is fine, they won't disappear again.