When did this problem start?
Did you add any new hardware or Software beforehand?
What do you mean it doesn't start? It won't boot into windows? A bLue screen? A Black Screen? No POST?
Have you tried the barebone test?
Unattach everything except CPU, RAM, video, Front panel connectors, 4 Pin power to motherboard and, 20 pin Connector. Then turn it on, If it POSTs correctly you will hear one beep. If it doesn't report back. if it does continue to add one component and turn it on until it no longer works.