Don't know for sure. Assuming you have an Award BIOS, that usually means "I can't initialise the Video Display!"
If you have one of these fancy BIOS where you can hook speakers into the onboard sound, it might tell you for sure(In a "Hu Man Like" voice, as my mobo manual says

).
However, not being able to initialise the display can be for a number of reasons, ranging from:
No Video Card (e.g. Loose video card, no video card, overheat, short etc.)
Where's My RAM Chip? (e.g. SDRAM loose/shorted/not there, etc.)
OMG My CPU Is On Fire! (e.g. Heat sink lost full contact, fan failure, CPU not in properly, etc.)
Cat Hair Shorted Out My 12v Rails. Help Me. (e.g. loose cable/bad contacts, shorts)
Dude, I Can Only Run At 100MHz, Not 400! (e.g. running a 100MHz FSB Duron at 133. Not that I have ever done this. La la la...)
EDIT: Damnit I hate it when this happens - I go and make a detailed post and someone else goes and solves the bloody problem!! *shakes fist*
