A couple other things to try if the card isn't loose:
1. If the board has onboard video, you may need to turn it off. This is done either with a jumper or in the bios. (It probably says something like "onboard VGA".)
2. Remove the PCI card and if the system works again, you might have to go into the bios and set what's typically called "primary vga bios" from AGP to PCI.
3. A primary PCI video card should typically be placed in the slot closest to the top of the board. (In some cases this doesn't matter.)