Probably a loose bulb, loose plug or a combination. One bulb at a time while following the wiring, ensure each bulb is properly seated. If any bulb looks discolored, it should be replaced. There are also tiny fuses in the plugs that should be replaced if they appear burnt/discolored. Replacement bulbs and fuses probably came with the tree.

Last year I had a similar predicament...only 2/3 of our prelit tree lit up, the whole middle section was dead. I had such a hard time tracing the lights throughout the tree to find where the problem was, that I had my son and his girlfriend cut off ALL of the lights. I ended up with a non-prelit tree, and just strung my own, and actually was much happier with it. (I was able to use LED versus the regular older style that was on there, and didn't have to replace the tree..nor throw out a perfectly decent tree)

Pre-lit are great...when they work. When they don't..what a pain in the rear!

I got tired of all the hours I spent each year trying to find which bulbs were defective. I finally got a gizmo called a Lightkeeper Pro and it helps a lot. Bulbs typically have a bypass built into each one which is supposed to pass the current on even if the bulb fails. However, this does not always work, and then you have a bunch of lights out and you aren't sure what has actually failed. The Lightkeeper passes a small current through the lights and usually succeeds in activating the failed bypass. Then you can see which bulb or bulbs doesn't work and replace just those. Below is a link so you can see what this actually does. It has gotten a lot of positive reviews on Amazon, and I think you can buy it for less than $20. That's a little pricey, but it can save lots of time.

Here is a link that might be useful: Lightkeeper Pro Video

With so many different styles/brands of lights out there, it is so hard to find suitable replacement bulbs..if you run out of the ones that come with the lights or the little baggy that's attached to the wire gets lost. I've bought replacement lights only to find out they don't fit, and then after all the work of trying to find the one "bad light"..I still am left with a non-functioning strand of lights and end up replacing anyway! Are there any universal replacement lights?

The bulbs only come in a couple voltages.

The bases are the problem.

Often you have to remove the bulb from the base and re-use the plastic base with a replacement bulb of the correct voltage.

I hate mini-lights.