how to fix christmas lights without a tester

Well, if you don't have a tester, you can try using a multimeter instead. Just make sure to connect the positive and negative terminals correctly, and check to make sure the light bulb isn't burnt out. If the bulb is intact, then the problem might be with the wiring, so you'll need to check each connection and make sure they're all secure. If all else fails, it's best to call an electrician for assistance.