I just got my new motherboard. Replaced the processor (1.1 GHz AMD Duron), installed RAM (two 256MB DDRs), optical and hard drive, tested the PSU (voltage connection green wire to black wire), and replaced the CPU fan. Now, I plug the monitor connector to the VGA card and power cable into the power supply. I power up the computer, the computer runs smoothly, except for a small problem: I see no video. I do not have a spare card to test if it's the card, but I'm pretty sure it worked before and I did nothing to it. Any ideas guys?

Binary search: undo half your changes, try to boot it.  If it fails, undo more.  If it suddenly works, reapply some changes.  Repeat until you find the change which broke it, then don't apply that change. :)

I think what Kp said would be best, make sure you have no jumpers wrong as well (that's what has gotten me a bunch of times) read the motherboard manual very throughly and try to see if theres anything you missed. Check the cord and make sure no pins are bent etc.. Also is your VGA card integrated? If it is make sure there is nothing in the AGP slot (By default it tries to display what is in AGP even if the plug isn't connected there). Anyway's that's all I can think of. I think I ended up replacing my mobo last time but I know after that I found out a whole bunch of new information and felt like an idiot :p.

This might sound dumb but make sure the video card is snuggly fit into the slot that it's supposed to go in. A similar situation happenned to me awhile ago, where i could not get video from a ATI card with an AGP slot. So i took it out, put it back in, then it worked. Weird.

Try installing the drivers for the card first using the on-board VGA, and then afterward, try the card again.