If you haven't done this already:

- Pull every card out of the computer, if any. Every. Single. One.

- Remove all keyboards, mice, sound devices, USB devices, everything.

- Remove all RAM sticks.

- Disconnect hard disks from the motherboard.

- Plug only the VGA cable into the onboard VGA port. Nothing else is plugged into the computer except the power cord and the VGA cable.

- Then, see if you get any video output when you turn it on. The computer won't "work" but you should at least get a BIOS message on the screen complaining about a lack of RAM.

If you get any video output, then power off, begin adding items back in one at a time, and power on again with each item, until you reproduce the problem with a single specific item.