On iOS after I'm leaving the application (pressed the home button) and opening it's once again after short period of time - camera interface will not be shown. If I'm leaving the application and waiting ~5 seconds and then return back - everething works as expected.
I'm using Unity 4.5.5 with Vuforia 3.0.9.
You may check this link for a solution
https://developer.vuforia.com/forum/unity-3-extension-technical-discussion/ar-camera-becomes-black-second-run