Just happened to me playing the new Lego Star Wars game. I was playing and then all of a sudden the system just died. I thought maybe the plug had come undone, but I checked it and it was fine.
I tried resetting it in the back and the little red light comes on but then when I hit the power button on the front, it tries to turn on and the little green light flashes for a second and then just dies again.
I guess I'll have to call Sony.