iPad 2 camera only shows black screen

After I take pictures and switch to mails, I note that my iPad 2 camera suddenly no longer shows image. It just has a black screen with the normal options. But the shutter opens and the device doesn't freeze. How to fix "iPad 2 Camera turned black" issue* Bring up the taskbar by double-clicking the home button, then hold the Camera app on the taskbar for 3 seconds, then press the '-' in the top