I had this problem with my keys before, (E53 i'm assuming?) both stopped working but not together, 1 went so I switched, and then the new key stopped working (buttons), I had the dealer look at it and, they found out that some actuator had failed or wasn't working properlly, and basically they said that the part that charges the key while its in the ignition was not longer charging the key, that could be another possiblity, they fixed it in 1 day rather quickly