newer parallel circuit type, and there is a short or break in the wiring. An easy way to tell if they are strung in series is if the wires at the bulb socket only go into the receptacle and come back out the other side.

Most likely your lights will be run in parallel, where the wires will also bypass the bulb socket. This is so if one of the bulbs go out, the rest will continue to light up. If you purchased your lights after 1995, they are probably run in parallel. Here's an example photo.

                                                    Lights strung in parallel

Before reading on, make sure the fuse in the plug end is good and not blown, and also insert a night light or other AC powered device into the socket to make sure you have power at the outlet.

Troubleshoot parallel run lights- Parallel run holiday lights are the newest type of lights, almost any set you buy now will be run in parallel. It is usually two 50 light series strands run together in parallel. If you have an entire light strand out, then most likely it is a wire that is shorted or broken. If it were just a bulb out, then the rest would still be working properly most likely. First look along the strand for any obvious damage to the wiring, if you find damage, fix it and that should do it. If you see nothing wrong, here is one way you can Ohm check your lights.

Touch your black meter lead to one of the male plug stems on your light strand.

Make sure your lights are unplugged before proceeding! Now connect your red meter lead into the female plug socket slot on your light strand (this is where you would hook up your next strand) touching the inside metal contact. You should get 1 Ohm or less in one socket slot, and around 30 to 100 ohms in the other slot with all the light bulbs inserted. With bulbs removed, your total resistance will climb, and eventually you will not have any continuity. If you get very high resistance or an open in both slots, then you have a break somewhere along the light strand, or you may have missing or inoperable bulbs.

Repeat steps 1 and 2 switching to the remaining male plug stem.

The last step is to check for a short. If while performing steps 1 and 2, you found that you have 1 Ohm or less in both female plug slots, then the wiring is shorted together. It is probably safer and easier to just toss a shorted light set in the trash and purchase a new one, as shorts can be difficult to find along light strands.

Troubleshoot series run lights- These can be a real pain to troubleshoot considering none of the lights will be illuminated, so finding the blown bulb can be tricky. I would first recommend laying the string end to end along a soft carpeted surface. Now quickly inspect the wiring to make sure there is no obvious breaks. If you find a damaged wire, fix it by cutting the damaged section out evenly and splicing together the new joint. If you found no damaged wire, you now have to check each bulb on the string.

When a bulb blows it sometimes leaves a dark or discolored surface. I would look along the lights for an obvious blown bulb and replace it. If you see no obvious faults on your light strand, then at this point you would have to replace each bulb one at a time, and see if it will light up after each bulb change. Of course you will have to decide if this would be worth the time, and I would certainly consider purchasing a new set of holiday lights instead.

Quick AC power check- Another quick way to test if you have power all the way through the light strand is to take a night light or some other AC powered device that will fit your light plug, and connect it into the far end of the strand with the other end plugged into the wall. If it lights up or operates then your wires should be providing a normal path for electricity.  

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