Well sounds like your video card might be dead, odd timing I have to say but there are a few things you can try.
1.) Load into safe mode by press F8 on system start-up. Once into Windows Safe Mode go to your start menu and go to run (or type into the search bar for windows 7). In either of those type "MSCONFIG" capitals not needed
Once in there go to the second tab called "BOOT". From there you will want to check mark "No GUI Boot" and "Base Video". The first option will disable the windows load screen with the progress bar (optional to disable but can cause odd issues on a monitor thats not the correct res or doesn't support it) and the important one is the "Base Video" which disables your video card drivers and uses the base video drivers from windows or the devices BIOS (your video cards). The second setting will force your system in 640x480 resolution, just go with me on this one
2.) Do this next step with the one above. Look up your motherboard model number and find out where the CLR_CMOS jumper is. This might also be called the BIOS RESET switch or CMOS RESET in the product manual. Clear the CMOS using the method described in the manual which is generally jumpering the pins for 10-15 seconds with the power still on and the battery still in. IF you cannot find those, you will want to pull the power plug off of the computer, pull out the CMOS battery (little round watch battery) and then press/hold the power button a few times to make sure you clear all residual power that may be in the system. Leave this off for 30 seconds to a minute to be sure it clears. You can then pop the battery back into the battery slot.
On your first system startup, you will more than likely prompted to enter into the BIOS by pressing something like Delete, F1 or F2. Go ahead and do that and correct the Date/Time as its going to be way the hell off. You should now try and see what happens when you load into windows using your add on video card. If you still get no signal, then plug the monitor cable into the on-board and go under Device manager in windows to see if it recognizes your video card. If so, then you may have to fiddle around with windows settings to get it to recognize it.
Your last option if it recognizes it in windows but wont show a picture on that video card is to go back into BIOS and find the setting referring to default display or video adapter. Sometimes these are set to IGP (Integrated Graphics Port) and you will ant to change the primary over to PCI-E or PCI Express, however its labeled.
Make sure after doing all of this that once you get display back to your video card (If you do), to go back under MSConfig and un-check "No GUI Boot" and "Base Video". If you don't un-check base video, your going to be enjoy a sweet 640xs480 no matter what you do.