How to ..
If you press the start button on the computer and nothing happens at all then, Huston, we have a problem!
Check that all the cables are pressed in correctly make sure that you have mains power at the socket you are using by plugging something else into it to test it and try again. If there is a noise or a light comes on but still nothing further first try the steps in It Starts But It Runs Really Badly. If you still get no further after all this then you have a hardware problem and will need to take a trip to your nearest Apple Mac repair shop. Here in Byron Shire it is Lightforce.
There is not a lot you can do if you have a hardware issue like this. What you can do is eliminate all other things like cables and connections so that you don't drag it down to the shop and find that it starts perfectly! This does happen...