My son has a Motorola Moto G. While running a Android system update the Moto G would not power on. How frustrating. I found the following remedy that worked:

1. Plug it into the charger.
2. Hold the VOL DOWN key.
3. While still holding the VOL DOWN key, press and hold the POWER key.
4. Hold both keys down for over 120 seconds. This is more than two minutes and will seem like a long time. You might want to time yourself to make sure you hold it longer than two minutes.
5. After holding the keys down for more than two minutes, release them.
6. The Flash Boot screen will display, and the Normal Reboot option will be highlighted.
7. Press the VOL UP key and the device will start a normal reboot.

If you have tried the above and it didn’t work, try this:

• Plug in the phone for 15 minutes.
• Proceed to Step 2 above.

I hope that Motorola releases an update that fixes this issue.