Hang in there.
So you built your own computer, way to go! It sounds alot more than upgrading your processor.
When it POSTs, does it beep?
So the new motherboard, have you installed the latest and greatest drivers for the board?
When you loaded the OS, did the video work then? IF it did while your building, but it doens't know that the OS is loaded, update the drivers.
Double check you cable connections? IF your video cable is attached to the MB, then we know you have integrated graphics, so it isn't as easy as a missing video card.
So the PC make and model, what was it? What did you change it to? What OS are you using?
Did your hardware class teach you about REQUIREMENTS? Did YOU check to make certain all of your components(hardware) have met the REQUIREMENTS for your project?
Please post back, we can make it work!