Murray has an AppleTV 4K and it's not working with his LG TV, unless he reboots it. Leo says it's probably an HDMI handshake issue. Maybe a bad HDMI cable. Or worse, a bad HDMI port. Apple says to hold the menu/volume buttons down for 5 seconds. The AppleTV will run through resolutions until one wakes it up.
David calls in to recommends getting Geffen Doctor HDMI. MonoPrice has a similar product. The HDMI can fall asleep and it has trouble waking up. Doctor HDMI keeps your HDMI ports awake.