When I put my arm down my apple watch goes dark. When I lift it back up, it stays dark. How do I fix this?

Make sure Wake on Wrist Raise is turned on in the Apple Watch app on your iPhone. The setting can be found in Display & Brightness settings. Also make sure the watch is NOT in low power mode. Wake on Wrist Raise is deactivated if the watch is in low power mode, which you can also check in the Watch App on your iPhone > Battery. Finally, check to make sure Theater Mode is not turned on, which you can check by swiping up on the watch screen to open Control Center. If the two mask icon is on, turn it off.