Well, this is odd. Can you still enter into BIOS? Or does it not even get that far?
If not, attempt a CMOS reset. There are guides online everywhere, and its quite simple.
Edit: Okay, I'm going to have to go one some assumptions here. For one, I'm assuming you are replacing the integrated graphics from your motherboard. If so, have you plugged in your monitor into the new gpu? If so, disconnect it and plug it back into the integrated graphics port. Boot into BIOS, and disable your integrated graphics. Save and exit. Disconnect the monitor and plug it back into the new GPU. Problem solved.
If you hadn't connected the monitor into the new GPU slot, and you are having issue. Plug it into the new GPU slot.
If you had already done all of the above, and still nothing, unplug the machine and conduct a CMOS reset. It'll take about 15 minutes, 14 of which is usually just waiting for all power to subside in the system to make sure it was truly reset. If that doesn't work, let us know.
Second problem. This is assuming you are replacing an older or different brand (AKA, ATI with an nVidia) GPU. If this is the case, you needed to uninstall the old drivers from the system, and THEN install the new ones. Since you can't get video, I can't help you do this. Normally though, you would boot into safe mode to uninstall the graphic drivers.