Back/Lap button does not respond

The LAp / Back button on my Swim 2 stopped responding after a workout. Now I cannot back out of a screen - If I get to the widgets screen, I have to shut down to get back to watch. If I select an activity I cannot get back to watch. If I change data screens, I can not go back. 

The lack of a back button makes the watch functionally useless.

Washed the button, soaked the watch to no avail. Software is current: 3.10