Are you replacing one AGP card with another AGP card? If you are going from a PCI card to an AGP card you may have to change a setting in the BIOS to tell it your VGA card is in AGP instead of looking for it on the PCI bus. Also, just to be safe, be sure to set your AGP settings to simple 1x AGP in the BIOS until you get things working. You can turn it up later once things are running again.
Assuming you are swapping one AGP card for another, I'm guessing you either are not seating the AGP card correctly, or you bumped something else in there loose.
Very carefully re-seat that AGP card, and make sure it is completely in the slot without any of the copper traces on the sides showing. Don't put the screw in it yet. Now check your other cards and your RAM, make sure none of them has been bumped. Now fire it up and see what happens. If it works now, watch carefully when you put the screw in to hold the card, sometimes if things aren't set exactly right in the case, tightening the screw will pull the card out just enough to cause a problem.
Give it a shot and see what happens.