All cameras are connected to video-in ports of recorder with cables and recorder is connected to a monitor with a cable, but there is no live video of all cameras shown on monitor (You can still see recorder's setting panel ) .
Please try below methods.
1. Check if you connect system correctly. Here is method.
2. Please go to main menu---system---general to change video standard to have a try. If it doesn't work, change it back to initial one.
3. Use hands to cover the front of camera. If you can see infrared lights turns on with red lights, cameras are on.
If all cameras are not on,
3.1 connect adapter to one camera directly without 1/4 spliter because maybe spliter is broken.
3.2 connect adapter of DVR to one camera directly without 1/4 spliter because maybe the cameras' adapter is broken.The adapter of DVR can be used to connect camera.
4. Restore recorder to factory default.
5.Go to main menu--system--digital--channel type. Make sure the first option is ticked.