Why my nexus 5 camera can't be reach sometimes?

Sometimes when I open the basic camera app on my nexus 5 , it showed a message says " can't reach the camera now " .. And the camera doesn't work even if I used another camera apps , it will work only after I restart my phone , and after some hours , it will happen again .

I had the problem on android KitKat , and still having it on Lollipop