Phone no longer turns on.

It's my brother's phone. He used it during the day without any problems. Now, half a day later, it suddenly can't turn on anymore. He doesn't think it hit anything or anything like that. He used it moments before I picked him up in the car. And he carried it in his (jacket) pocket all the way home. Hours later when he tried to use it, it was off and couldn't turn on.

It appears just like if it's off, and when you try to push the on-button, nothing happens. No lights go on, nothing.

If you try to charge it, it doesn't even light up, as I think it used to do.

We've also tried taking the battery out and hitting the power button, to drain power in the circuitry, and then reinsert it, and try again, without luck.

We don't have a spare battery to check if that's the problem. But it could still hold many hours of charge earlier this day.

What could've happened? And is there anything to do to fix it?