Does your graphics card make a beep/whine sound when you turn on the computer? If so, the power is not properly connected.
If your graphics card makes no error noise, it could be that your BIOS is set to initialize PCI graphics before PEG (PCI Express Graphics). If you have no way to get into the BIOS config, you may need to use an old PCI graphics card just to get into the BIOS. It's also a good idea to check with a PCI card just in case your problem lies with another component.