Electric

Insid or outside? Likey a bad ground.

I'm going to make an assumption that you're talking about the clearance/running lights on a towable rig.

First, check all bulbs in the circuit that is not operating. Not knowing your model, the bulbs could be wired in series and if one goes out on that circuit, they all go out.

If all the bulbs are good, you might have a loose connection. pull each light fixture out and see if you have voltage coming into it and going out of it. T If not track back towards the tow vehicle and determine where the open circuit is.