One fan of CPU isn’t working
Either your connector cable is faulty or your Fan. Connect the other Fan with the connector where other fan is working, if it works connector is faulty otherwise Fan is damaged.
It's okay to use one fan but it would be best if you are using two fans. I mean, fan provides cooling and one will have to take more load. Sometime heating could be more and beyond the cooling power of one Fan. It's always better to use two Fans then one.