DIAGNOSIS This is almost always a HDMI handshake or video format mismatch between the Apple TV and your TV or A/V receiver. The box is working, but your TV isn't understanding the video signal it's sending.
FIX You need to force the Apple TV into a safe video mode. Unplug the HDMI cable from the Apple TV. Turn on your TV and set it to the correct HDMI input. Now, plug the HDMI cable back into the Apple TV while it's on. This often re-initializes the handshake. If you see a picture, immediately go to Settings > Video and Audio > Format and change it to a basic setting like '4K SDR' with Match Content turned on for both settings.
IF THAT DIDN'T WORK Try a different HDMI cable, preferably one certified for high speed. Also, try plugging the Apple TV directly into your TV, bypassing any soundbar or receiver. If that works, the issue is with the middle device, and you may need to adjust its HDMI settings (like turning off HDMI-CEC or using a specific input).
HEADS UP This model supports HDR10+ and Dolby Vision. If your TV is older and doesn't support those, having the Apple TV set to '4K Dolby Vision' permanently will cause a black screen on many sets. That's why using '4K SDR' as the base format is the most compatible starting point.