What to do when AppContentPage can no longer be found in Xamarin.Forms?

I upgraded to the latest version of Xamarin Studio and now my project won't build and I can't set my iOS app as the startup project. Not even an option anymore. To add insult to injury, AppContentPage can no longer be found in Xamarin.Forms. How do I fix those two issues? Any ideas? Sorry, folks - pretty frustrated. This happens often with Xamarin Studio - things break for seemingly no good reason after upgrades or package updates.

Xamarin Studio 5.9.8 Build 0

Xamarin Forms 1.5.1.6471

Thanks and Best,

-Auri