Why is my Apple Watch screen black?

I also forgot about one thing about why your watch screen is black:

• Your watch is powered off. Press the side button until the Apple logo appears, then release it.

There are some possible reasons for why your watch screen is black:

• The battery on your watch has died. You may need to recharge your battery to turn the screen on. This might take a few minutes.
• Your Always On setting is off. To turn it on, go to Settings > Display & Brightness > Always On. Toggle the switch to turn on the setting.
• Your watch is in Low Power Mode. If you’re running watchOS 10 or later, press the side button, then tap the battery percentage. Click the yellow “Low Power Mode On” button to turn Low Power Mode off.

Also, one thing to note, it is called Apple Watch, not iWatch.

Sometimes, a lot of times, the customers have to teach the Apple staff. Another reason your watch screen may be going black, is you might've changed your workout to low power mode. This has been happening to me in my swimming app workout, that I set up in Activity. So, obviously we have to learn on our own.