Since a recent Unity update (pretty sure it’s the one that added the ability to scale the gameview) I’ve been seeing an intermittent issue where the game view will just go blank.  This happens in both Edit & Play mode, but only on my Mac.  Initially, I had to quit and reload Unity to resolve it, but discovered, reselecting my window layout from the ‘Layout’ drop down restored the game view.  So it became annoying and not as aggravating :D.

I’ve been following a thread on the Unity forums where this is being discussed:

http://forum.unity3d.com/threads/blank-game-view.421817/#post-2772116

I tried the suggested start-up flag and the problem went away, though I noticed a performance drop in the editor, but nothing serious.  I used the following command line from a terminal window to start Unity with the flag set.  (Note, I have my copy of Unity in a folder that includes the version name, you might need to change that):

/Applications/Unity5.4.0/Unity.app/Contents/MacOS/Unity -force-gfx-direct

I hope they find & fix this one soon.