you key's battery may be dead, but try this first:

-gather all your keys
-open door, get in, close door
-first key: turn to position 2 in the ignition (on position) five times quickly
-take out key, hold up (unlock) button, and press bmw (lock) button three times
-release, doors will lock and unlock to confirm success
-repeat for each additional key

a replacement key from the dealer will cost about $180-$200

__________________