You don't mention what year of BMW this is, but I would check under the rear seat, or in the trunk, as well as the driver's side fender well, there may be a splash panel there in front of the wheel covering the battery housing.

the battery is in a little cubby near the right rear wheel in the trunk, lift the cover in the trunk and there you go there's the battery, you can also boost your battery from under the hood, on the firewall on the passenger side of the engine compartment theres a postive terminal under a plastic lifter, Also the problem might be that your central locking system might be triggerd which shuts everything to do with power off. it's located under the drivers side dash nere the clutch pedal. hope something helped