key for renault megane not working, car won't start?

Does anyone know how i can get the code to start my reanult megane. The central locking wont work resulting in me not being able to start the car. There is still a slight signal in the key as i had it checked. Will i need a new key?? the chasis number is 18085078 if that helps to get a code
any help would be greatly appreciated.
Louise