Humidity found its way in. Sadly none of us knew this, but Versa's water proofing fails after 10 or so months and after that period, swimming, showering with it or even washing dishes or sweating is likely going to kill it.
Contact Fitbit support. You are in a similar situation as I am since my Versa 2 (bought in Dec 1st last year) died last week after exercising and it's clear that there is sweat inside it. If you want to confirm what happened, put a bit of ice on the back on top of the sensors for a few seconds and then watch if condensation forms inside.
