Soaked urad ki dal too long - Is it safe?
If it was soaked in the refrigerator for that amount of time, it should not have a very bad smell, and should be safe.  However, if it has been sitting at room temperature for that long, and has developed an unpleasant smell, it likely has fermented, and should not be risked.  While there is no guarantee that it is yet harmful, it is not recommended from a food safety standpoint.