i want to know which products you are using for the cameras and nvr?
I want to know if you have tried to view the camera on the web interface
Nevertheless you could troubleshoot the following:
ok, As regards the cameras not showing on the display
1. check if the cameras are still powered by physically examining the cameras while connected to the switch,
2. if you are using a POE switch, connect your laptop to the switch and try to see if you can assess the camera through the web interface
3. Check also if ip address of the cameras is not conflicting with another
4. Test the cable.
5. if everything seems ok yet its still not displaying images but if you can find the camera on the nvr list, then I suggest that you also check the nvr configuration, if it was hikvision , their NVR usually have where you can specifically assign a camera to a particular display channel (menu-configuration-live view)