The title says it all really. I got all excited about the new release of Visual Studio and so I upgraded my copy of Visual Studio 2015 Community Edition immediately. The upgrade has some nice new features (and I want to use it to write Xbox One programs) but it did do one thing that confused me for a little while. It clears the source setting for Nuget.
The result is that all your Nuget fetches fail. However, it is easy to put back again by going to Tools>Nuget Package Manager>Package Manager Settings and setting the checkbox as shown above.