Starting on Thursday, May 11 we started seeing intermittent issues with downloading NuGet packages on our primary environment.
While we are investigating the root cause make sure your builds are using nuget 3.5.0 which is installed by default on build worker VMs and available in PATH. NuGet 3.5.0 has better support for timeouts/retries management. Also, to retry nuget restore use `appveyor-retry` script like that:
appveyor-retry nuget restore <path-to-solution>