Why does my computer not recognize video card?

Here's what happened, i bought a computer. The power supply was not getting electricity. I reconnected the cables and booted but the cpu/fan would turn off after like 20 seconds. I turned it off and on again and reset the bios. Then i reconnected cables again so it would not turn off. I finally got the cpu/fan/power supply all to work but now i can not even see anything on my monitor. The video card and monitor work. I tried putting an agp and a pci video card that both work but nothing comes on the monitor. Everything seems fine, but i do not think the motherboard is detecting the video cards since i set the bios to default. I have a a7n8x-la motherboard with an amd athlon xp 3200+ cpu. Any ideas on what to do? I can not change the bios since i cant see anything. If anyone could please help me i would really appreciate it.