IPhone 4s fell, and turned off, working now?

My iPhone 4s fell no more than 2 feet onto a wood floor, flat on its back. It was in sleep, but after it fell, it was booted off completely. So first question, how does a physical impact boot it off? Is that a feature, or did something break inside?

Now, even after holding the power button for 3 seconds, it didn't turn on like it normally would after botting off. I tried more, and I don't know if hitting it or if holding the power button for longer did the trick, but it eventually did turn on and is working like normal right now. So what are the possibilities? Is there maybe a loose piece in there now that could result in it shutting again on a more minor impact? Is it safe to use like normal, since it seems to be working?

My 3GS has fell more than once. But is normally working. Compared with 4S, the back of 3GS is more than firmer. I think the Iphone is very firm. You can shake you iphone, If there are not any strange voice, go on using. Just take care of it next time

_I would like to TravleChina.HK

It should be ok to use it
my story with my 4s was that it had a otterbox defender case on it and i put it on top of the unused grill cover and it slid and fell about 3 feet to the concrete on its back. I got scared and picked up my phone and the screen was blue and all of a sudden started to boot off. I'm not sure if it booted back on automatically or if i held the power button to turn it back on, but the phone worked just fine with no problems. That was when i had the phone for about a month since this june.