Check Power Indicators
When having problems with security cameras and monitors, the first thing that you should do is to make sure that each camera or monitor that you are having problems with has its power indicator light on. If the light is not on, make sure that the unit is plugged in to a power supply and that it is turned on. Should the unit be battery operated, try replacing the batteries and seeing if the power indicator light turns on. If the unit is plugged in but still is not showing power, attempt to plug it in to a different outlet or power supply to see if it will power up there. Checking to see if the unit has power is not only one of the quickest ways to solve some problems, but it will also let you know quickly whether your problem lies with a faulty connection or if it is an electrical problem which will either require you to repair or replace the unit.
If your security cameras and monitors are connected via coaxial cable or some other form of video cable, check to make sure that all of the cables are correctly connected to the various units in your security system. While looking for loose connections you should also keep an eye out for signs of corrosion such as white or green scaly buildup on the connectors. Disconnect the unit from a power source and scrape loose any corrosion that you might see and tighten any loose connections that you might find. Have someone keep an eye on the monitors while adjusting the connections on cameras that are not working, as they will be able to tell you whether the cameras begin to work again or not.
If your cameras connect as part of a wireless computer network, log in to the router that runs the network and see whether the cameras are appearing in the router logs. You should have an IP address or a MAC address for each device and computer on the network; if one or more cameras don't appear, use a signal detector to make sure that there is a wireless signal that the cameras can use. If a signal is present, reinstall the drivers for the cameras so as to make sure that the computer running the system can recognize all of the cameras on the network.
Replace Wiring and Components As Needed
If after checking the connections and ensuring that the units are getting power you are still having problems with portions of your security system, begin swapping out possibly faulty cables and components with units that are known to work. Plug in a working camera where a non-working camera is, checking to see if this solves the problem. Replace lengths of coaxial cable with cable that has been proven to work before to make sure that the problem doesn't lie with damaged or otherwise faulty cable. If a monitor doesn't work, try attaching a new monitor and seeing whether it gives better results. These solutions are much more work-intensive than the previous ones, and therefore should only be started when the simpler solutions have been tested and found to be insufficient.