Side Button doesn't work

After surfing with my Apple Watch Series 4 44mm a side button stopped working. The button itself clicks all right but the Apple Watch doesn't respond. Other than that my watch works fine. See YT video https://youtu.be/zxXA6kv7Ypg.

Restarts doesn't help.

How it happened:

I was surfing with my Apple Watch with Surfing Activity Workout on. Screen was locked.

Several times my watch got paused by itself 😠. I had to click two buttons simultaneously (a side button and a crown) to unpause it. This also makes a screenshot of watch face.

Suddenly I observed that Emergency Services screen appeared 😠. I'm still in the ocean. I can't bring back Workout app because Apple Watch is locked and buttons do not respond. Unlocking my watch in the water to enable buttons won't help because screen is wet and my fingers are wet - taps are not recognized 😡 I continue surfing.

Then after some time Surfing Workout screen was on again.

When I finished surfing and went from the ocean I found that my watch battery was finished.

I came home and put the watch on magnetic charger to charge it. It started to charge but after some time a black screen with Apple logo appeared. It is unusual, it has never been so before. Buttons were not responding. After some time the watch turned on and continued charging - green digits showing time and green circle with charge percentage appeared.

When charged 100% I put on my watch, tapped passcode and found out that the side button doesn't work. Apple Watch doesn't respond.

What could happed? How to fix? Restarts doesn't help.

Youtube video https://youtu.be/zxXA6kv7Ypg