Apple Watch shows a red “!”

Assuming you read the related article link at the top of this page and the suggestions didn't work, then the last part of that link tells you to contact Apple Support. This is not Apple Support. This is a user to user only forum.

You should schedule an appointment at your Apple Store Genius Bar for service options on your watch --> Genius Bar Reservation and Apple Support Options - Apple

There's either the procedure called Forced Restart as detailed in the article OR Apple Service. There is unfortunately no magic bullet in between.

You're welcome and good luck.