This must be a localized error because I didn't find anything in particular for NET-DVR Server application and its available errors. However, based on the above info, it appears that you can't connect or the Net-DVR Server is offline. I recommend you start it and check the IPs of the cameras. If it's possible connect to each camera individually with a network cable to diagnose the problems.
In most of the cases, a simple reset of the camera and the system should get thing backs. If you're using a Firewall and port forwarding, make sure they're configured properly.