video loss

It happened all of a sudden at 5a.m in my shop 2 days ago. I was outside and no one was doing anything to the video recorder or cameras,so it happened all by itself. Since then Instead of video stream ,I only see dahua black logo on all 4 squares where video stream should appear. Video footage...

I'm trying to setup the email notifications in my NVR for Video Loss and Video Tampering. We occasionally have cameras quit working so I'd like to know sooner than getting phone calls that the cameras are out. We have 30 cameras spread between 3 buildings; separate Hikvision poe switches in...