On Android, make sure you allowed camera access when asked (Epic Torch uses it only to control the flash LED, never to take photos). You can change this in your phone's Settings under Epic Torch. On iPhone, camera permission is not needed.
Close other apps that may be using the flash or camera, such as the camera app or another flashlight.
Some phones and iPads have no flash LED. The on-screen flame still works.
If the phone is very hot or the battery is very low, the system may refuse to turn on the LED.
Send the app to the background and come back. After that, tap the torch to relight it.