Power but no startup!

I have a Duron 900 running Windows XP and it has been working fine until recently. I turned on my computer 2 days ago and suddenly it won’t start. All the LEDs on the case are on (power, hd activity…even cd-rom leds) but the computer just doesn’t start up…there’s just a blank screen and my monitor reports that there is no signal. I have no idea how to even approach a problem like this so if anyone has any idea, it would be much appreciated. If you have an answer please e-mail me at TheCARaisins@new.rr.com Any response would be great, thanks!