Whole sections or individual bulbs? also are these LED of incandescent?
LED's should last 5 to ten years of continuous use .. incandescent 3000 hours or so.
If it is whole sections I would check the connections between each section. power down and unplug. then use the tip of a paper clip to "rough" up the contacts and plug them back in
I agree with KMS on this. Most rope lamps are LEDs encased in a plastic tube preventing them from getting wet so they can be used outside. Depending upon the length of the chain of lights you need to check each twist connection point for corrosion. Unplug the lights, and disconnect each section from the other and clean the connection points both male and female points. Once done reconnect and you should be fine. If not one of the many bulbs in that section have failed. If that is the case, it cannot be successfully repaired.