First check your motherboard manual and/or bios to see if you have on-board video that needs to be disabled so that your cards get the signal.
Then also in the bios make sure that the Video Seek at bootup is correct for either the AGP or the PCI slot.
Save and Exit.
Make sure that you have a good connection for your cable from the monitor to the video card (and that, if you have on-board video, that you don't have it connected to the wrong video output).
Pull the card again and make sure the slots are clear of dust bunnies that would cause a crap connection when the card is inserted into the slot.
Make sure your AGP card is not a PRO type card that requires the cover to be taken off the last notched area, and if you don't have a PRO card, make sure that the last notch is still covered with the plastic piece that should be in place for a regular AGP card.
Make sure that your power supply cord for the monitor is attached firmly at both ends.
Reset your CMOS jumper on the motherboard to OFF for a few minutes and then move the jumper back to the ON position. Some people say power up the computer for a second with it in the OFF position, and others say not to power up at all. I've done it both ways to reset my CMOS/BIOS with no ill effects.
Last...borrow a monitor from somebody long enough to test to make sure it's not your monitor that has bit the dust coincidentally at the same time you opened your case.
My computer was woking fine until I took it apart to determine the type of hard drive I had. I plugged it back in and turned it on and the montior was blank. I have an AGP video card so I switched it out with another AGP card, and still no signal. I thought the AGP slot might be bad, so I put in a PCI video card, and still no signal. I also checked the monitor with another computer and it works fine. Could my motherboard be bad? It is only 2 years old, it's an IBM. Any suggestions would be great.