Depending on your vehicle, in the "old days" when one bulb when out on the circuit, everything went out on that circuit. We'd start tracking it down by checking the fuse, the look at each bulb that uses that fuse. And every time, one was blown. I had one car that when the brake light bulb blew, the radio quit working too. Another one, when one of the bulbs blew, the dash lights went off too.
So, not knowing how your vehicle is wired, I'd double check this idea first.
Another thing, more than likely your trailer tail light uses the 1157 bulb, which has 2 filaments in it. If both are out, it's probably the ground. I had a utility trailer once that drove me insane because one side kept going out completely. No brake, no turn flash, no running light, all 1157 bulb. Every time, it was a broken ground to that bulb socket, which was actually in plastic and had a separate wire connected to the trailer frame.
I would check that second.
