Start with the basics:
1. Do you know your power supply is good?
2. Do you get any boot up screen or beep codes?
3. Do you know your motherboard is good?
4. Do you know your CPU is good?
5. Do you know your vid card and RAM are good and inserted well. Try re-inserting both of them.
We would need a lot more info about your system, what symptoms it shows, and what you have already tried to help.
Good luck.
