Could be a gazillion things really.
Here's a TUTORIAL to get you started in the right direction.
You may also want to have the car's computer scanned for trouble codes, to give you further clues.
One simple thing you can do is squirt some starting fluid in the air intake (NOT while cranking), and then try to start the car---if it kicks over briefly, then you know you have a fuel delivery problem. If it won't even give a hint of starting after introduction of starting ether, then it's probably an ignition or electrical issue.