If you are not getting display output from SHIELD TV, follow these steps (in exact order below) to enter Display Safe Mode:
1. Connect SHIELD to your TV via HDMI.
2. Connect SHIELD controller to SHIELD using the included USB cable. Use the USB port nearest to the HDMI port.
3. Plug in SHIELD to a power source using the power adapter provided in the box.
4. Immediately, hold Buttons A and B together at the same time on SHIELD controller until you see the Fastboot menu.
5. Select the option to boot into Safe Mode. You can navigate the menu using the X and Y buttons.
The display resolution on boot should now be safe native TV resolution mode for your display (generally 1080p at 59.94).
After you see the display on your T, check for and apply any software updates in Settings > About > System updates.
You can also change default display resolution settings via Settings-> Display & Sound -> Advanced Settings.
To exit Safe Mode, simply reboot your device.
Note: When in Display Safe Mode, HDCP is disabled so apps like Netflix won't be able to play content.