DIAGNOSIS HDMI-CEC (called 'Control TVs and Receivers' on Apple TV) is a finicky standard. A power cycle, TV update, or new cable can break the handshake that lets the remote control your TV's power and volume.
FIX First, reboot the entire chain. 1) Unplug your TV and Apple TV from power for 2 full minutes. 2) Plug the TV back in, turn it on. 3) Plug the Apple TV back in. Let it boot. Often, this re-establishes the CEC 'conversation.' Then, on the Apple TV, go to Settings > Remotes and Devices > Control TVs and Receivers and make sure it's turned ON.
IF THAT DIDN'T WORK The issue might be your TV's CEC setting, which often has a different name like Samsung's 'Anynet+', LG's 'SimpLink', or Sony's 'Bravia Sync'. Dig into your TV's external input or system settings and turn the CEC function OFF, wait 10 seconds, and turn it back ON. Then retry the Apple TV setting.
HEADS UP Some HDMI cables don't reliably pass the CEC signal. If you recently changed cables, try switching back to the old one. Also, if you're routing through an A/V receiver or soundbar, that device must also support and have CEC enabled, which adds another point of failure.