XamarinForms-error CS0246: The name of the type or namespace 'X' (missing a using directive or an assembly reference?)

Today in the morning I had this problem and I have not been able to solve it, I understand that the error CS0246 indicates that I am missing some reference, I have tried to add and remove the reference of the application but I have not obtained nothing ... and I do not know what to do. This happened to me when I rebooted my computer for some reason.

I already tried to erase the cache, recompile, clean the solution and before I sent a thousand errors, but these are the only persistent.

//MainActivity.cs

using System;

using Android.App;
using Android.Content.PM;
using Android.Runtime;
using Android.Views;
using Android.Widget;
using Android.OS;
using Xamarin.Forms;

namespace DatePickerService.Droid {
  [Activity(Label = "DatePickerService", Icon = "@drawable/calen", Theme = "@style/MainTheme", MainLauncher = true, ConfigurationChanges = ConfigChanges.ScreenSize | ConfigChanges.Orientation)]
  public class MainActivity: global::Xamarin.Forms.Platform.Android.FormsAppCompatActivity {
    protected override void OnCreate(Bundle bundle) {
      TabLayoutResource = Resource.Layout.Tabbar;
      ToolbarResource = Resource.Layout.Toolbar;

      base.OnCreate(bundle);

      global::Xamarin.Forms.Forms.Init(this, bundle);
      LoadApplication(new App());
    }
  }
}