I have had an HP z558 media-center for almost 2 years, and recently there is no video on the monitor even during the boot-sequence. I had it configured with the DVI output to LCD TV, and VGA output to monitor (in clone mode).
I have done a cold-boot many times and it seems to boot properly with the front-led welcome message and auto-connection to the Wi-Fi network.

I suspect that it's a video-card problem and was wondering whether anyone knew how to further diagnose and fix the problem. The specs says that it's an nVidia GeForce 6600 NV card.

Did you ever find a solution to this issue with the HP z558 and VGA/DVI not working? I have the same problem.

Thanks