The Fitbit is waterproof, but it seems hot sweat can work its way in and short things out.
I use my Fitbit while playing roller derby. It takes a bit of a beating and is the second one to suffer at the fate of my attempts to keep fit.
This one went weird. The touchscreen stopped working, and the heart rate sensor stopped registering my pulse.
Having nothing to lose, I opened it up. What I found was... not click bait at all. It was pretty surprising though.
The seal around the case must have failed, allowing sweat into the watch.
On the right you can see the salt left from my sweat. It's also present on the PCB.
To fix this I carefully rinsed the affected part of the PCB with isopropyl alcohol and allowed it to dry.