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.
I'm experiencing same problem. DVI output failed, switched to vga which still worked, but now vga has failed! In both cases, the failures were progressive in that output might be normal at first but then go black (often with a period between when image would show various crazy artifacts).