This is a known issue with nuget version 3.4, which is included with Visual Studio 2015 Update 2:
The NuGet.config file in your
%AppData%\NuGet\
folder has accidentally been emptied. To fix this: Close Visual Studio 2015, delete theNuGet.config
file in the%AppData%\NuGet\
folder and restart Visual Studio. A newNuGet.Config
file will be generated and you will be able to proceed.