Back button stopped working

The same thing Happened to Me. I think it was caused by swimming in a pool but not rinsing the Watch off afterwords. Or I could have just been unlucky (Water resistant doesn’t mean Waterproof) and the water found its way in.

If the issue is just a dirty button you may be able to bring it back by rensing off the watch with clean water, but it could also do nothing or make it WORSE if water damage is actually the issue. Otherwise you will have to open the watch up and replace the Back/Home Ribbon Cable with a new one and hope that solves it (The issue may be on the Mainboard). If you go this route you should then always treat the watch as if it is no longer even slightly water resistant.

Its not as difficult as tearing apart a modern Phone, but the parts are much smaller and if your still able to get use out of the watch it may be best to just live with the issue/Get a new watch.