In the game, the camera has been moved.
I hope when player press a key, he can reset the camera back to original position.
The only way I know, is use Game Actuator -> Restart Game.
But I don’t want the game restart, just reset the camera’s location is OK.
You might find it easiest to add an action that sets the location back to the start, and play it as necessary.
just do not forget to deactivate the action after playing the single frame 
If you use play mode on the action actuator it deactivates automatically when reaching the end frame. If you use loop end mode and a keyboard sensor it deactivates on button release.
When I think about it, using a one frame action should work with any configuration.
I had a short look at the mentioned thread. It deals with orbiting. So I suggest you set an orientation key too. Otherwise the camera might look just somewhere.
Thank you very much. I solved it.
Beacuse the Camera has been parented to an Empty, so I add play action to empty, not camera, then it works.
Thanks all again,