A second hand watch? If so, the battery may be dead. Otherwise, I'd recommend performing the Master Reset to factory defaults with deleting all data and settings. That should help disabling any options that may cause higher than usual battery drain. If it is Instinct Solar or Instinct 2, then also make sure you disabled the PulseOx option - it consumes the power a lot. The same may apply to some Connect IQ watch faces (just in case of Instinct 2, Instinct 1st gen does not support Connect IQ).
