If you are still using Visual Studio 2013, you may notice that today, the Package Manager Console will have stopped working, any attempt to install a new package will give an error like
The source at nuget.org [https://www.nuget.org/api/v2/] is unreachable. Falling back to NuGet Local Cache at C:UsersyouAppDataLocalNuGetCache
This is because NUGET has withdrawn support for TLS 1.1, and VS 2013 will use TLS 1.1 by default.
To Fix this, type this into the PM console: