After changing the glow plugs on my BMW 320D I cant start the engine .What can be the problem

Did you disconnect the battery? You might have to get some computer work done, its an issue with BMW.

try disconnecting the battery, turn lights on then off to discharge the system, reconnect battery. because if you disconnect any components within a live system you can input codes which will prevent the engine from starting Is this even relevant? have you checked the battery charge state? computer controlled starting shuts off the circuit if the voltage falls too low. Sometimes new components need to be initialised by using a computer interface. if this is the case you will need someone with a suitable code scan to sort it out. You dont say what year it is, or why you need to change pre heaters?

I'd also like to know what it was that caused this? If