battery problem

the car was switched on and i forget to remove the key from the socket after 3 or 4 ours i came back to the car i tried to start it but it won't start i think the battery is dead after some trails even the key is not coming out of the socket
please help me
what should i do and how?
or if i need to replace the battery which is a good battery
the car is 2007 BMW 740Li
thank you