If you see the LED lights flash when the camera application is open, you may have had the flashlight the iPhone lit since the last time you used it and you forgot to turn it off. This can sometimes happen in broad daylight when it is not dark enough to see that your flash is still on, and that usually increases the battery drain. To solve this, simply turn on the flashlight from the Control Center.
If the flash does not turn in any way, and see no logical reason, verify that you have placed the iPhone in any hot spot or have used the terminal in a long video recording. In certain circumstances, the iPhone is configured to disable the LED flash when it gets too hot, as it avoids excessive overheating. Once the iPhone is cool, everything should return to work perfectly.