I assume this is a wireless backup camera system. Have you tried moving the camera closer to the monitor?
If you have and there is still no signal, maybe the camera is switched off, it isn't getting power, maybe it has more than one channel, or it just isn't working.
Class C, 2004/5 Four Winds Dutchman Express 28A, Chevy chassis
2010 Subaru Impreza Sedan
Camped in 45 states, 7 Provinces and 1 Territory