I have audio but no video on my Apple TV
I have audio but no video on my monitor from Apple TV
Apple TV 4K (3rd generation)
What screen are you using with the Apple TV box?
Is that a different screen as used for setup or when it last worked?
Do you get no video at all, or only when streaming content?
Is this for using apps on the Apple TV box (which one), or when using AirPlay streaming?
If you get no video at all, then see the troubleshooting steps listed here: If your Apple TV doesn't turn on or shows a black screen - Apple Support, especially step 5, to cycle between various video settings, until you find a valid setting.