A non-working button (when all other buttons work) usually means the watch is broken and needs to be replaced. As a last resort you could try soaking the watch in lukewarm water for 30 minutes, but this is risky if the watch's water resistance has been compromised due to broken button.
Same issue here: Forerunner 235 running flawlessly for 4 years, then the start/stop button also stopping operation within a few days. The watch is now rendered useless. I'm struggling to believe that *all* watches mentioned here would all have the same button failure ...
Sounds like a common problem for those who use their 235 in watersports. I’ve had mine for 4+ years, wear it daily/nightly and have had very little trouble with it, until this weekend. Was in the ocean and the wetsuit pushed the Start/Stop button and now it doesn’t work. Powered it off, by holding the power down for 15 seconds and the Start/Stop button worked once. Have tried soaking it in Luke warm water, to no avail.
Anyone find a solution? Can’t imagine there isn’t a way to clean the contacts.