Welcome to Android Central! Do you remember what the battery was at around the time it died? Was it draining fast, or was it still pretty full?
The two main possibilities are a failing battery or an even worse problem with the motherboard. Try plugging the phone into a wall outlet overnight, without disturbing it. When you try to power it on, press and hold the Power button for at least 15 seconds. If it still doesn't turn on, you probably need to contact a Motorola service center, or bring the phone to a repair shop.
10-24-2014 10:18 PM