Unity Camera Depth

Unity has this concept of Camera Depth.  It's got nothing to do with the zbuffer, it's the order that the cameras get rendered if you have more than one camera active.   Somehow my main game camera got set to a camera depth of -1 when I added another camera which meant that even if it was the only camera active it didn't get rendered.  So that's an hour I can never have again. 
If your camera mysteriously stops working despite the fact you can see it working just fine in other camera views...  this could be your problem. 

https://answers.unity.com/questions/418471/camera-preview-is-okay-but-game-view-is-not.html