If your NVR is showing video loss on all camera's, please reboot the NVR, this will connect to all the camera's again.
Once all the camera's are displaying images once again, you will need to adjust your camera's bit rate and possibly frame rate.
The cause of the NVR losing all its camera connections is the network card is being over loaded and the maximum bandwidth is being exceeded.
For the best bit rate and fps settings of your camera's when used with the NVR, please set them to match this help topic - here
So what is the maximum bandwidth I can use on my NVR?
The maximum Mbps you can use on your NVR is 48Mbps, please see here