How can I find a loose neutral wire?

You need to give us more details. Is this a home circuit, for lighting or for receptacles? Start your search by checking all the white wires in your panel, every screw must be tightened

I got power coming from the panel box to several lights but they don't work. I've tested the power in the lights an switches an there is power. Somebody replied that there must be a loose nutral somewhere in the line. Its really confusing to me. I work construction an have done some wiring an this is so confusing. I've checked the brakers, wires, an the panel box. Just one day the lights stopped working, there are also other lights on the same braker that still work, the house is old an I don't think that there is a brake in the line, cause like I said that there is still power to the switches an lights. I also changed braker, light fictures, an switches.

So how are you confirming power is reaching the lights, what wires are producing 120vac?

My suggestion would be to pull a couple of the nonworking outlets to see if the "quick connect " holes in the rear were used to wire them. These are notorious for developing poor connections after a period. Rewire using the screw terminals. The bad connection will probably be at the first nonworking outlet or the last working outlet. You may as well rewire them all. Otherwise it only a matter of time before another bad connection develops.