Seems to boot, but not display

Hello,

I have an odd issue. I am a computer technician. I sold a bareboned kit built computer that I built myself to a client. A year and a half goes by with no issues. But now I have a problem. When the computer is booted up, all lights come on, fans come on and everything sounds and looks normal, except there is no display on the monitor. I swapped monitors and am using a known good one. I reseated everything, all connections are firm. I replaced the power supply and ram with known good hardware, no luck with the problem. I finally decided to gut it and replace via warranty RMA the motherboard and ram. With a new mobo and new ram, the problem is the same. The CPU is the only thing that hasn't been replaced because it is no longer under warranty. There are no cards in pci slots and nothing else I can think to try. I was leary of buying a new processor because the return policy won't let me return the new one if I take it out of the box and it doesn't solve my issue. Any suggestions?

Thanks!