It's a battery problem, which is not good and can not supply enough power to turn on the flash. So when you take a picture with flash mobile no longer has power on and off (or perhaps some automatic off or something).
What you have to do is buy a new battery, there are cheap on Amazon for example. My passed me on my S2, especially when I had multiple applications open. In addition, always happened when the flash was turned on for the second time.