It sounds like your Fitbit got overheated (more likely this time of year than too cold if you are in the northern hemisphere). Hopefully it will recover in a few more hours once it has cooled sufficiently. I would recommend taking it off and putting it to one side out of the sun (not in the fridge! - it is not supposed to get too cold either!).
Sense, Charge 5, Inspire 2; iOS and Android
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