This subject comes up often. Be really nice if you could post camera gui settings, NVR settings, etc. so someone may use this as a guide to replicate.
I understand you arn't sure why it started working, but back tracking and posting settings would be helpful and should allow you to understand what changed/occured.
Ok, I finally figured it out and I guess it wasn't a minor miracle after all. :/ What happened was I loaded the config tool for Dahua cameras onto my laptop as someone suggested and then once the software found the camera, I changed the IP settings for the camera to the IP settings that the GUI said the camera should be. However, the camera still wasn't working so I looked at the GUI settings again. They were on PnP and Hikvision with a port of 8000. So, I changed those settings to Manual, ONVIF, and port 80. But it still didn't work so I changed the GUI setting back to PnP and FINALLY, the camera started working. The GUI also kept the ONVIF and port 80 settings which it had never done before (it always changed the settings back to HIKVISION and port 8000).
I also just added another Dahua camera to the system using the above directions and the NVR finally added the camera connected to Port #2 as Camera/Channel #2. I was worried Channel #2 had burned out for some reason.
So, every port/channel and camera is correct (camera #1 is plugged into port #1, etc) except that camera #7 is plugged into port #8. Weird.
Finally, the iVMS now doesn't even see the 2nd Dahua camera. Maybe it has a problem with Dahua cameras?
And I will try to get pictures up soon. Thanks everyone for all the help! I hope this helps someone else as well!