Hi Dave
When charging your watch, check that:
All plastic wrap has been removed from both ends and both sides of the charging cable (check closely on the head of the charger).

Under normal operation, charging should only take up to around 2.5 hours for a full charge. If the battery level is low, leave your watch charging for this length of time before using it. After that time, remove your watch from the charger. If it has not already turned on, try restarting or - if that doesn't help - force restarting it:
- Restart : press and hold the side button until you see the Power Off slider, drag it to turn off (wait until it does), then press and hold the side button until you see the logo.
- Force restart: press and hold both the side button and Digital Crown for at least 10 seconds, until you see the logo.
More information:
Check your battery and charge your Apple Watch - Apple Support