My car is BMW , myself I replace the battery’s key , now can open the door , but doesn’t start the car .why?

is this your only key if so you have damage your transponder chip on your circuit board so take your lock cylinder out of your column you need the key to do this and a bobby pin and remove your antenna ring and your ews you need to replace your ews and transponder that is in the key so to get ews and transponder key that are correct with the lock cylinder if car is registered to you take key to dealer or maybe the car and have them cut and program key for your car your problem is do to ews is not communicating with your chipped key it is your security

don't listen to this fool it literally could be a dozen reasons its not starting so don't run out and buy a new cas