Its the motherboard/processor/power supply. Try the motherboard first! I had to replace mine and they gave me a better motherboard, a better processor, and a better gfx card! for a discount :D
- If the fan has power, it's not your power supply unless you added new hardware, then theres a possibility.
- If the CPU LIGHT goes on, but your monitor doesn't, its your monitor. Look at your monitor light!
- Your motherboard is responsible for your RAM, GFX card, Processor, and everything else! I call it the core.
- Dust could've clogged up the PC. Clean it up!
- If all of these fail, disconnect all hard drives and all CD & DVD drives. If it boots up, you know your problem. Just reconnect one by one, following with a startup.
This answer has a 90% chance of answering your question correctly.