Apple Watch not restarting.
Held button on right side for 15 seconds and still not restarting. Totally dead
Question marked as
Which Apple Watch are you using?
Has the battery ever been replaced?
To force restart your watch, you will have to press and hold the Digital Crown and the Side button together for about 10sec, until the Apple logo comes back again.
More troubleshooting can be found in this article:
If your Apple Watch won't charge or it won't turn on - Apple Support