I don't know the maintainer, but it looks like maybe the powerup's server isn't responding. I suggest waiting a day or so and see if it comes back. Hopefully it is just a temporary problem related to the hosting provider.
If you urgently need some new fields you could try out Amazing Fields or Trello Custom Fields, but those may not have all the features you want from Smart Fields. The best option is probably to wait around over the weekend and see when it comes back up.