Why 2002 BMW X 5 wound start with the original BMW key but lights and everything else works?

It sounds as if the ECW chip inside you key is bad. this is the chip that talks to your car and tells the vehicle that the key is the proper key. The cut of the key is right so it will unlock the tumblers, and let you turn the ignition switch, but the car needs that signal from chip to tell it to start. you will need to replace the key at the dealership. about ($120-$190). The keys on websites wont work they may be cut right but only BMW has the key with the proper signal to start the car.