If you have comfort access, there is a replaceable battery in the fob... battery available at any place that sells watch batteries
If you dont have C/A... then get in the car, close door.. stick fob in key holder slot and push it in all the way and HOLD it in for a few moments...
this re-programs the fob to the car.. then eject it and try the locks..
have you tried your other key ???? if the above procedure does not work then you need a new fob which is covered under warranty
'11 128i M-Sport Pkg, Sport Susp., 6AT w/paddles, BMW Perf CAI,Charcoal Filter Delete, BMW Perf. Big Brake Kit, BMW Perf. Exhaust, Ferodo 2500's, Titanium shims,135 trunk lip, BMS-PBX Box-Map 3, Castrol RBF600 ,e93 FSB, , f30 Backing Plates,GC Camber Plates, Hankook RS-3's (Track setup)