What's wrong with my Defender 21020 DVR?

[dk1000 0]

7 year old Defender DVR. Model # 21020. The cameras that came with it are good. All the wiring and connectors that came with it seem good, as well as the cable for the Ethernet connection. I use NetViewer 2.0 on my PC to view and control, and all 4 camera squares are black, and will keep saying "Network connection failed". Often I will not even be able to log in and will say the same message, and when I can log in, again, all 4 cameras are black. However, every once in a while will get a current frozen image from all the cameras. Can't review any footage, will get the same network message if I go to Playback. The red light that normally flickers when it's recording , just stays on steady. Also all the name, password, IP addresses, ports, etc. are blank on the Netviewer 2.0 screen. However if I connect using the A/V inputs on my TV, I can see all the cameras live, but the wired mouse is frozen, wants my user name and password, but again, can still see live cameras and any outside motion with the login screen on top of it, which I can't log in to. But after a moment, the Defeneder logo will come on a black screen, and then again, the cameras will show with the frozen mouse, and repeat. The system did start working again for about a day, then now stopped again. DVR will make a short beep sometimes, and sometimes a long sustained beep. My IP address didn't change, and I use DynDns to relay the cameras. I assume what goes thru the A/V inputs to my TV is a direct live feed, and what I see on the PC using Netviewer goes thru my network and Internet? Would like to find out if indeed network related or if the hard drive is fried, so I know which to work on. If the hard drive is fried, are DVR's fairly universal if I can't get an exact brand and model replacement? Thanks!