Uh Oh...... if you've changed the battery and reprogrammed the key (which you must have done to get it to work the central locking), and now find that the car doesn't start....
Sounds to me like the little black transponder that sits inside the key could be missing.....
Remove the 'blade' part from the main body of the key, and on the inside of the plastic bit attached to the blade there should be a little black box, this is the transponder. Without it the immobiliser will just cut the fuel to the engine.