Does the engine turn over? Does the engine turn over slower or faster than normal? Do you hear the fuel pump prime the fuel system when you turn the key to the on position? Have you checked to see if the timing belt is intact? Are you getting fire to the plugs?
If it's not turning over check all the connections at the battery, ground and at the starter and make sure they are all clean and tight. If you don't hear the fuel pump check the inertia switch to make sure it isn't tripped. If the timing belt is still on check to make sure it hasn't sheared off some of it's teeth. If you don't have fire going to the plugs test the coil and crankshaft position sensor to be sure thay are good. Check all these things then if you still have a problem post back the results of your testing.