Hi,
I can only assume from the lack of details about your PC that you have a video card installed and you have your main monitor plugged into that with a second monitor plugged into the mainboard's video card. If that is the case and your main monitor can display the BIOS screen, and then nothing, but the secondary monitor is working, then it is your card that is not working correctly. This could be caused by the driver or a faulty card. First try uninstalling the driver and rebooting and let it reinstall on reboot. If that will not work, go the the card manufactures site and download an updated driver and install that. If that will not work, I would be looking at replacing your video card.
I am having a mental blank and I can not think where you do it but you should check which monitor is active after boot, it could be that windows is only using the secondary monitor.