I spent the weekend Updating my Visual Studio 2015 installation to Update 3. And I installed the Core .Net CLI (Command Tools Interface). And rant the update "ASP.Net and WebTools tooling".
I get back to work, start Visual Studio, open our project, and now I get this:
The 'NuGetPackage' package did not load correctly.
The problem may have been caused by a configuration change or by the installation of another extension. You can get more information by examining the file:
'C:\Users\samuel\AppData\Roaming\Microsoft\VisualStudio\14.0\ActivityLog.xml'
Restarting Visual Studio could help resolve this issue.
Continue to show this error message?
Here is the error from the Activity Log:
CreateInstance failed for package [NuGetPackage]Source:
'mscorlib'
Description: Could not load file or assembly 'NuGet.Tools, Version=3.4.4.1321, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
or one of its dependencies.
The system cannot find the file specified.


System.IO.FileNotFoundException:
Could not load file or assembly 'NuGet.Tools, Version=3.4.4.1321, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'
or one of its dependencies. The system cannot find the file specified.
File name: 'NuGet.Tools, Version=3.4.4.1321, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

at System.Reflection.RuntimeAssembly._nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)

at System.Reflection.RuntimeAssembly.nLoad(AssemblyName fileName, String codeBase, Evidence assemblySecurity, RuntimeAssembly locationHint, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks)

at System.Reflection.RuntimeAssembly.InternalLoadAssemblyName (AssemblyName assemblyRef, Evidence assemblySecurity, RuntimeAssembly reqAssembly, StackCrawlMark& stackMark, IntPtr pPrivHostBinder, Boolean throwOnFileNotFound, Boolean forIntrospection, Boolean suppressSecurityChecks) 

at System.Activator.CreateInstance(String assemblyString, String typeName, Boolean ignoreCase, BindingFlags bindingAttr, Binder binder, Object[] args, CultureInfo culture, Object[] activationAttributes, Evidence securityInfo, StackCrawlMark&
stackMark)
 at System.Activator.CreateInstance(String assemblyName, String typeName)

at System.AppDomain.CreateInstance(String assemblyName, String typeName) 


WRN: Assembly binding logging is turned OFF.

To enable assembly bind failure logging, set the registry value [HKLM\Software\Microsoft\Fusion!EnableLog] (DWORD) to 1.

Note: There is some performance penalty associated with assembly bind failure logging.

To turn this feature off, remove the registry value [HKLM\Software\Microsoft\Fusion!EnableLog].
</description>
<guid>{5FCC8577-4FEB-4D04-AD72-D6C629B083CC}</guid>
Uninstalling and reinstalling Nuget Package Manager from Tools -> Extensions an Updates seems to have fixed it.
We'll see. Someone on another post said they thought it worked but then came back and said it didn't: VS2015 - The 'NuGetPackage' package did not load correctly