I'm assuming that you were able to power on the camera and monitor, and saw what you expected to see from the camera. Now, the monitor and camera aren't talking to each other. Is this correct?
I'll assume that your monitor is in place in the truck and connected to a live power source. The monitor is reporting that it can't find a camera. Correct?
I'd start with power to the camera. Most cameras get their power from the marker lights in the center of the trailer up by the roof. In other words, those lights have to be on in order for the camera to have power. Are those lights on? Do you get an image on the monitor? If the lights are on, but no image, I would try to pull the camera out and see if it is actually connected to power. Sometimes the connection is there when you are getting ready to install the camera, but when you push the extra wire back into the hole something lets loose. If you are lucky, it lets all the way loose and the problem is obvious. The fun comes when the connection either isn't completely tight or the wire is broken inside the insulation. It may work perfectly while pulled out away from the trailer, but the connection fails as soon as you try to stuff the wire back into the hole.
Does your camera use a repeater that is mounted on the roof of the trailer? If so, check that it is firmly connected. Some of those repeaters use replaceable batteries while others need a hard-wired connection.