PC doesn't recognise any GPU

[Adrian98765]

My older system which is a OEM HP DC 7700 with a Core 2 Duo E6300 and 2GB RAM doesn't recognise any GPU.

I've tried
Radeon X1600 Pro
GeForce 9500GT
GeForce 8800GT
GeForce GTS 450 (my gaming gpu that i'm using)

None of them works. The onboard GPU works fine.

I was told to disable the onboard GPU through bios, I've searched all the bios, it doesn't have such option. My gaming pc has this option, but this doesn't. Any ideas?

 

[LaborinjoRt]

Is the motherboard slot itself dead? How old is the motherboard? Was it damaged or stopped working at any point?

 

[Adrian98765]

xD I don't know if it's dead. Last time (2 weeks ago) the 8800GT I was using for it worked fine. I've been using this pc for 1 year. I don't know how old is the mobo, but the integrated GPU chipset is from 2006. It suddenly happened while I tried to power on the pc, blank screen.