Grayed out, eh? Does the Weather app on your watch have permission to access your location? On your watch screen swipe down and tap the Settings icon, tap Apps and notifications, scroll to and tap App permissions, scroll to and tap Location, scroll to and tap Weather, tap the All the time setting.
If you have allowed the Weather app location permission and it is still grayed out, I would try to reset your watch to see if the that fixes your issue. On your watch screen swipe down and tap the Settings icon, then tap System , scroll down and tap Disconnect and reset, tap the okay Checkmark (right). Check to see if your Weather app is now functioning properly and no longer grayed out.
If a reset doesn't bring the app back into service on your watch, you may need to do a factory reset.