Just a couple days ago, I encountered a desktop PC that beeps which point to a no video situation. But the problem is that the video adapter is onboard and I did not brought a separate AGP card, so I cant confirm if its the onboard video adapter that is the causing the problem.
I try to trouble shoot to best of my knowledge and use my technician's instinct,when I reset the BIOS setting on the mainboard, the display was restored. I suspect that the primary vga display in the BIOS, was wrongfully set to a different device(let say PCI or AGP, when it suppose to be set on its onboard video) causing the blackout.
Next time when you encounter such a problem, try resetting its BIOS to make sure that its not a case of wrong BIOS settings.