Motherboard won't detect GPU

[H0R53]

I've got an Optiplex SFF machine I've been tending to for some time now as a side project. Current specs:

8GB DDR3 1066

Intel Core 2 Duo E8500

Stock 235W PSU

640GB 7200RPM Seagate Momentus

CD drive

Stock Motherboard

nVidia GT 620

 

However, I replaced the heatsink because I was going to upgrade to a Core 2 Quad Q9400, and now it won't detect the GPU.

Here are some examples:

Usually, when I have the VGA cable on the onboard port with the GPU installed, the BIOS complains that I 'have an add-in GPU and need to use that'. Now it doesn't. I plug the VGA cable into the GPU, and I get nothing but a black screen.

 

It worked fine this morning. The GPU fan still spins up, and I've tried other GPUs with no success, as they work fine in other computers.

 

Does anyone know what the problem could be? I really don't want to buy another motherboard for this thing.

 

The onboard video works fine.

[Verrierr]

Check if there's anythign jammed in the PCI port. Did you have to remove the graphics card in order to swap heatsinks?

[H0R53]

Just now, Verrierr said:

Check if there's anythign jammed in the PCI port. Did you have to remove the graphics card in order to swap heatsinks?

Yes, I had to remove it as well as the motherboard. I cleaned it out with a plastic card and an air compressor. Haven't turned it back on yet.