I have had this problem in the past, and know how insoluble it seems.
My advice is, don't be panicked into buying new bits just because someone says,"this happened on mine, it was XYZ, I fitted a new one, and now it's fine".
There are a lot different potential causes, as Rhair says, but remember most non-starting problems are caused by something very simple, not a component failure.
Post what year and ignition/fuel type you have.
You need to approach this systematically.
Carefully search for all the connectors and fuses you can find, remove each one, clean with the approved fluid, waggle them back together again.
Procedure then varies depending on system - I assume it is fuel injection not carburettor?