The first thing to check is to make sure that all the bulbs are intact and not broken. Next, you'll want to make sure that the lights are connected properly and that there are no loose wires or connections. If that doesn't solve the problem, you can try replacing the light bulbs with new ones. If that still doesn't work, it may be time to invest in a new set of lights. Finally, make sure that your tree is receiving enough power to run the lights.