Friendly suggestion, always provide the model of your device. Different methods of troubleshooting may apply based on model type. Diid this occur while trying to update the OS?
-Be certain to try unplugging the power from the AppleTV, IMHO you should wait 15 minutes then plug it back in. Is it still rapidly blinking after plugging it back in?
--If your AppleTV is a 4K model and is under warranty (standard warranty to AppleCare+), I'd recommend taking this directly to Apple Support.
-If you are no longer under warranty and you are not on a 4k model, this is the only assistance document I know of:
https://support.apple.com/en-us/HT203028