apple TV fine one one TV but not another?

I am assuming you can't get the ATV to display on the original TV?

Since your ATV worked fine until one day and works fine on another TV, by basic problem solving, your problem is the original TV otherwise your problem would carry over to the other TV.

Something went wrong with your TV. If you have another HDMI source such as your cable box, switch it's cable to the TV input you used with the ATV. See if it works fine.

Also try the ATV on the input on the TV used by your cable box input If it still works fine.

Your TV input could have broken or the TV settings could have been changed accidentally causing the problem.

I assume by "will now work" means no display?

Do both TVs have the same resolution - 720p or 1080p over HDMI?

If the AppleTv has been set to work with a higher resolution on a newer TV or incompatible lower one, then you may get video on one but not the other.

Have you tried the same HDMI cable on both TVs ?

See the links here, especially for cycling video display resolution:

Get help with audio, video, or power on Apple TV - Apple Support

Welcome to the Apple Community.

What is it that doesn't work about it, could you provide more information please.

I have the same problem there is no audio but the video is clear and working.

What TV are you using (brand, model #)

Is it connected via HDMI?

Try ry a different cable and port

Test on another TV