My bmw 318 ran out of petrol now it wont start i

if you can't hear the fuel pump cut in for a couple of seconds, check the fuse. Check the relay, or replace it with a known good one. See if you have power to the fuel pump. If you have power, your fuel pump is in need of replacement.
When you run an electric fuel pump out of gas, you should always cycle it about 10 times before you try to start the engine. That is, turn the key to on for a few seconds, and then off, and repeat. You need to get some fuel up to the engine.