First thing to try is re-sync the key.
There is a button in the car which locks all the doors. On mine it was by the handbrake next to the hazard light switch, but it might be on the dashboard. Check your handbook.
Press this button and hold it in for about five seconds. The doors should lock and then unlock.
Release the button then, within 15 seconds, press the lock button on the key fob. The doors should lock and unlock.
Wait a minute then lock and unlock the car again using the fob. You should now be able to start the car.
biffo, February 2009