Depends on what system you have, but most back up camera problems are usually one of three things.
1. No power to camera because the vehicle isn’t hooked up with 7-pin connected and the tow vehicle lights aren’t turned on. (Surprisingly, this happens more often than you think).
2. Camera needs to be repaired with the monitor. Depending on the brand, most monitors have a RCA style power jack next to the cigarette lighter power cord to help provide power while pairing a camera with the monitor. Typically cameras ship pre-Paired.
3. Tow vehicle has LED lights, which are managed with PWM. PWM chops up the power into discrete bits, allowing for computer control of how bright the lights are. Unfortunately, this chopped up power wave form does not provide enough power to run the camera. There are dongle and adapters that include a capacitor that smooth out the power form such that the camera can be powered. Check your tow vehicle lights to see if this is the case. (Unfortunately, this is a real problem, and happens with more modern vehicles.)
I lied. There’s a fourth thing:
4. Camera is too far away from the head unit for a strong enough signal to be read from the camera. You can usually address this with a repeater that’s mounted closer on the trailer (often upfront) or an extended antenna on the camera and the receiver unit. Recognize that antenna length is tuned to the camera frequency.
Just my opinion. I could be wrong. Hope this helps.