I've been having this problem too, ever since the latest software update. It will be working one moment and then turn off the next. If this happens, I can't turn it back on unless I restart the phone. Then, it will inevitably happen again.
Something is causing this to happen and I guarantee you it is not the OS itself. You've more than likely got a bad app; your job is to figure out which...
even though i restart the phone it still does not turn it happens sometimes but today it just does not turn on i have tried to restart and turning it off waiting for a while uninstalled some apps i got the other day but still no luck
I had this problem too after I manually flashed Android 4.4 KitKat, had to do something with the radio. I just took the radio.img from the latest factory image supplied by Google and flashed it, and after that I've had no problems anymore.