Could be lots of things and depends alot on the age of the car is it petrol or diesel.
Start with fuel is there sufficient in the tank
If it is a petrol car take out a spark plug and see if it is damp with petrol after turning the enine over a few times.
If the above is the case then it may be an electrical problem ie no spark to spark plug.
Suggest you post again with answers to the above
Maurice, January 2010
