Firstly are you sure that the battery is not defective?
Try turning on the headlights and seeing if they stay bright.
If the battery is OK then I'd say that this sounds like an immobiliser fault, if you have been using the same key to start the car as you usually use.
If you have tried using a different key then this key may not be being recognised by the immobiliser. When you put the key in the ignition and turn on, does the red immobiliser light come on and then go out? If it stays on, then there is a problem that the immobiliser is not seeing the key ( which will have a small chip in it and is scanned by the immobiliser when put in the ignition barrel ). If the light is not on, then it could be that the engine earthing strap has become detached / broken or that the starter solenoid is faulty, in which case you would normally hear a small click when turning the key. If the earthing strap has broken, it is likely that the alternator light will not be on when the ignition is switched on and there will be no click either as the starter will be completely inoperative without an earth.
Steve J, August 2007