Christmas tree lights can be a bit finicky. There are a wide variety of reasons that your lights may not be working properly. The first is if they were not stored properly, this can lead to the lights becoming loose or broken over time. In addition you may have a burnt out light bulb on the strand. You should inspect the lights to see which bulb or bulbs appear to not be working properly. It's all a matter of going through each bulb to figure out the issue, which is time consuming to say the least.