1. How old is the system?
2. What are the system specifications?
3. Open the case and switch on the system... See if the Harddisk comes on (you can touch it to feel the components' rotation or you may even hear a low whirring sound)
4. Have a technician look at both motherboard and CPU. Motherboard may have developped bad capacitors or CPU may be dead...
5. If you have Win 2k/XP and your motherboard is faulty, try to replace with a similar model because of the system configuration... (otherwise, you may have to boot in safe mode and install new drivers)
6. Connect your Hard disk as a secondary to another computer and see if you can access it and if necessary, take backup of important files...
7. Verify if SMPS has sufficient capability to power all components (if you have additional cards such as gfx, audio, lan cards, and also for USB devices that power on through computers' USB port, you will be providing power thru SMPS)
Arun