depends on what key (if it is original or just a replacment from a locksmith) connect battery or jump leads to power car then place in door lock and lock and unlock 5 times if that dosent work then open car put key in igniton and turn on / off ignition light until it bleeps and ata last resort bmw dealer whitch will just plug in and reset full ecu probly cost bout £150 hopefully this will help
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.