The Related Article Link above which brought you here, has suggestions, such as let it sit on its charger for 30 minutes as well as try a Forced Restart. Have you done both?
A Forced Restart means to hold BOTH the digital crown and power button at the SAME time and keep holding until the Apple Logo appears, then let go.
If you're using the authentic Apple Cable, you've tried to just turn on the watch by holding in the elongated power button until the Apple Logo appears or the above Forced Restart procedure and the watch doesn't turn on, then you'll need to contact Apple Support for help.
Tap the Get Support button at the top of this page.