DIAGNOSIS The power supply is insufficient. The Chromecast draws more power than the USB port on your TV or a weak charger can provide.
FIX Unplug the Chromecast from power. Plug it into the wall using the original power adapter and USB-C cable that came in the box. Do not use your TV's USB port. Power it on.
IF THAT DIDN'T WORK The device may have corrupted during a software update. Perform a factory reset. Hold down the button on the side of the Chromecast for at least 25 seconds while it's powered on, until you see the Google logo on your TV screen. This resets it to factory settings.
HEADS UP The HDMI port on your TV may not supply enough power even if it works for other devices. The official power adapter is a 7.5W unit. Always use it.