We are going to need a lot more than that if you would like us to help you.
Start with a full parts list including maker/model of everything that you know inside of the case.
Secondly, the more you can describe about the background, the better. Has this computer ever worked and if so for how long did it work? If it ever worked, did anything change shortly before it stopped working.
Thirdly, the more you can tell us about what is happening with the hardware now, the better. When you hit the power button does anything happen at all? Does it try to start and turn right back off? Does it sound like it is starting and staying on and yet nothing happens?
If you don't answer questions like these then we can't possibly help you.