I found this in relation to this topic on a different site:
"All you need to reset is that media scanner: Settings > Apps > All > Media Storage: Force stop & Clear data. Then, reboot."
Most people seem to think this works. In a follow up, someone posted this:
"I had the same problem and none of these fixes worked. It turns it that my Spotify was the problem. It had crashed and could not find the media files i had downloaded. I uninstalled Spotify and that solved the problem"