That is the mode where it shows the clock without booting WatchOS. It suggests that the battery is empty. Place it on the charger for an hour and then take it off, hold the power button for 10 full seconds, and see if it starts up. If not, hold the power button for 30 full seconds, wait and see if it boots and if not, hold it again for another 5 seconds and then see if it boots.
If that does not work you'll need to bring it into Apple and they can run diagnostics in the store to check if it needs a new battery or if it's otherwise defective. They will tell you what it costs up front and you won't be charged any more than that.