- Reboot
- Check cable/try different cable
- Correct video input selected on TV? Try different video input?
- TV off?
- If going through soundbar, check input/ouput there? Perhaps try direct connect between AppleTV and TV to see if that gets you video & audio, then mix in the soundbar (if there is one) and this will narrow in on a specific link in the chain where further investigation/experimentation is warranted.
So many possibilities: share more detail including a list of what you've tried so far, like suggestions above.