Q: Hdmi-cec suddenly not deteced by ATV4 anymore

Try the following steps, check whether things are working after each step where appropriate, before trying the next.

1. Restart the Apple TV (Settings > System > Restart).
2. Restart the Apple TV by removing ALL the cables for 30 seconds.
3. Restart your router. (Also try removing it’s power cord for at least 30 seconds)
4. Reset the Apple TV (Settings > System > Reset > Reset all settings)
5. Restore the Apple TV (Settings > System > Reset > Restore)

Then I'd have to suspect your cables or other devices as the source of the problem, are you able to test the Apple TV with other equipment.

Then there's just other equipment left to test it with.

Test it with another TV or receiver.

You need to find out whether the problem is with the Apple TV or your other equipment. You could take it back to the store for testing but often it's more convenient to do it yourself, although if it turns out to be the Apple TV, you still need to go to the store anyway.

You might try it at a neighbours. You also might try it connected directly to the TV if you currently have it connected through the receiver.

I have an ATV4 which demonstrates numerous HDMI dropouts.  Video system reports "not found".  System confirmed as working fine with ATV3 - same cables, same system input.  Not finding a lot of reports of ATV4 HDMI problems, but I believe I may have one.  You might as well.

I don't have the problem and besides the problem is with HDMI-CEC not a loss of signal, it's a totally different matter.