If its not turning over could be a bad connection on battery / or a bad earth.
If it is turning over and not firing --
1) If the key light (on dash) is staying on, could be a bad connection on steering column for immobilizer. (try letting of adjuster lever and shaking steering wheel.
2) If the key light goes out, could be the Crank Sensor as suggested above.
On other cars I've had, when the crank sensor has gone, it hasn't put on any lights or thrown up any codes for the diagnostics to pick up on??