Iphone 4 SUDDENLY TURNED OFF

My iphone 4 suddenly turned off!
it had above 70% charge and just 5 10 mins before it got off, i was using it, a call had come i just picked and then rejected in 2 seconds. After that i left it on the back seat of the car and after sometime when i got it, it just wouldnt turn on.
i even put it on charge for like some mins and still it doesn't work.
its all BLACK. I also held the power button for more than 1 min and still no changess.
it wasn't wet or anything. I just don't know what suddenly happened

I had a similar situation because my car was really cold inside. Most of the time when electronics are left in te cold, they won't turn on. To fix this, I got a hair blow dryer and warmed up my phone. After a couple mins, I pressed the power button and it turned on! I guess it just needed to warm up.

Ha! Happened to me too.

When the iPhone crashes, locks up in a way that prevents it from being turned off, or just isn't working as it should (you'd be surprised at how many problems or strange behaviors can be solved by a simple reset) you need to reset the iPhone. Here's how:

Begin by holding the sleep/wake button at the top right of the device and the Home button at the
bottom center of the face at the same time.

Hold them both until you see the screen go black. In some cases, you may need to keep holding
them even after the red power-off slider appears. If it appears, just keep holding it.
Wait until the silver Apple logo appears.

When this happens, you can let go - the iPhone is rebooting.