turn the key and car won't start! lights and radio work...

Start with a new battery, Just because the lights and radio work doesn't mean the battery is good. Your battery has 6 cells of a little over 2 volts each and if one cell shorts out it will still power some things but won't have the amps needed to engage the starter

Take to dealer and reset the ECU it's a security thing with BMW cost bout 110.00.