Why won't my computer start up?

I turned off my computer and now it won't start up. Upon pressing the power on the following happens. It makes its normal beep sound. It makes it to dos. (that black screen with all the white writing) then it just goes black. The windows sign never pops up, when it should and pressing caps or num lock doesn't make the lights on the keyboard light up. I have windows xp. please help i really need my computer for school.