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