The Dark Sky API is officially dead (as of yesterday) and I'm recommending that users find a new API and use the Ghost XML plugin to get their weather data into Indigo (or use one of the other fine weather plugins).
Some users are switching to a "bootleg" API that emulates Dark Sky at https://dev.pirateweather.net/forecast/ in order to keep using Fantastic Weather. You can also find information here if you want to try this route.
But as of yesterday, I'm unfortunately no longer supporting the Fantastic Weather plugin.