Why would a Camera disappear from the Hierarchy on play?

My Main Camera is disappearing from the Hierarchy when I hit play. If I go to the Scene view, I can see my GameObjects moving around, but my Main Camera disappears when I hit play so I am unable to see the actual game.

Not sure how it happened. I was testing a function which froze Unity, had to End Task it and after I reopened my scene, everything appears fine but my Main Camera ceases to exist when I hit play.

I had the exact same problem in the first tutorial for the 2D UFO project. Closing Unity and opening the project again fixed it.