check all the obvious stuff first. Make sure your monitor isn't unplugged, or you vga/dmi cable isn't unplugged. Try unplugging individual components (I.E. video card, sound card, cd/dvd etc.) see if that changes anything. Or try resetting your cmos, either by a reset button on your motherboard, or taking the battery out. (had the same problem last week, and taking resetting the cmos fixed it for me)
You make any changes recently?