1. Turn off your SHIELD.
2. Unplug the power cable from your SHIELD..
3. Connect your SHIELD to your PC/Mac/Linux machine's micro USB port.
4. Plug the power cable back into your SHIELD.
5. Wait 2 seconds, and then hold the NVIDIA button for 5-6 seconds until you see the fastboot menu on the display.
6. Tap on NVIDIA button to move on Menu to "Boot recovery kernel"
7. Hold NVIDIA button to for 2sec to select "Boot Recovery Kernel"
8. You will see dead Android robot on screen. This is expected behavior.
9. Hold NVIDIA button to for 2sec to process to next menu
10. You will see Android Recovery Menu
11. Tap on NVIDIA button to move on Menu to "wipe data/factory reset"
12. Hold NVIDIA button to for 4-6secs to select the "wipe data/factory reset"
13. After process select "reboot system now"
14. Device will boot