Power off before POST

I have a PC running Windows NT 4.0 workstation. User reported that the monitor went blank and system would not start. After checking all the obvious -cables, power source, etc. I have: replaced the monitor, swapped the video card, purchased and installed a new video card. All attempts have netted the same result – at power up, the monitor power light goes to green but NOTHING comes up on the monitor, then the monitor shuts off again (light from green to orange). I have also tried to start the system with the WIN NT setup disk. Any suggestion(s) greatly appreciated! 🙂
Donna