We've seen this happen on rare occasions. You can try one of the following two methods to recover:
1. Hold down the power button for at least 10 seconds. The screen will stay black.
After releasing the power button, wait 3 seconds, then press the power button again for 3 seconds.
The tablet should power up now.
2. If the first method doesn't work, there is another possible button sequence you can do to revive your device...
For the nVidia Shield -
- Press and hold the Volume Down (-) button
- Press the Power button while continuing to hold down the Volume Down (-)
- Release the Volume Down (-) after 5 seconds or when you see a small menu appear on the screen
- Highlight the Continue option using the Volume (+/-) buttons
- unfortunately, the menu text is very small...
- Press the Power button once to select - your device will then reboot as normal!