My Apple Watch screen went black. Holding side button , or trying to force reboot, was not having any effect. It stayed black. It does charge and is on a charger now

You posted several hours ago, so your watch should be charged enough to recover by now. If it is still unresponsive to forced reset (press and hold both buttons at least 10 seconds) the only thing we can suggest is Contact Apple Support and arrange to have your watch checked at a Genius Bar appointment or official Apple Service Center.