Top right button unresponsive

My watch is stuck in pool swim mode. I can't exit it. I can't power off. As the button to select does nothing. It does press normally but nothing happens.

Was working perfectly last night.

Update: this afternoon the watch dropped out of the pool swim activity and was left with a green triangle on screen.

I followed the instructions to reset from there. Nothing seemed to happen for a few minutes and then it came back to the watch face.

However the top right button does nothing now, so I can't get into an activity or accept any change.

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