No display

I'm having a problem with a neighbors computer. It's built in video module went out so I installed a video card. It worked for a few days but then his mouse and keyboard froze up. Then when I got those working the display went out. I've tried two different AGP video cards, I even took the card out of my computer, but nothing worked. There is no problem with his RAM as far as I can tell so I'm lost. There are no beeps when it is started. I can't check the cmos because there is no display.