Whenever i play certain games it does this, but usually comes back afterwards. I have also noticed if I plug the device in and then unplug it, it generally comes back.
This happens by design, to make sure the screen is bright enough for you to see and manipulate, when the ambient light level is quite high, like when you're outdoors.
Just cover the light sensor for a few seconds with your hand (it's in the same place that the LED is) and you should see the brightness drop and the slider become enabled again.
Peter that makes sense, but I am usually on a train/metro when it happens to me. (the only time i play games on the device). It is usually fairly dim in those areas.
If you're in full sun it automatically jumps to full brightness to compensate and Greys out the slider. Similarly ij the dark it will decrease brightness. The light sensor is in the front camera.
I figured this out by getting irritated by my screen constantly brightening and dimming while reading in portrait - then I realized that I kept covering the sensor with my thumb. Oops.
