most likely alternator is fried. Car ran on battery for some time and then once the battery died so did your car. The battery light blinks when your voltage that the alternator puts out drops below a particular number (I think it is 12.5V). Had a similar issue with my car and had to have my alternator rebuilt recently. I caught it in time tho and didn't get stranded.
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--'00 BMW ///M5 .:Jet BLK/BLK:.Individual-Style 65 Wheels; 9.5" Square Set up;275 Tires;Spacers;Black Kidneys;Muffler Delete; 3.5" Dual Tips; Beast Power; "Dinan" Ducts; E60 SSK; BC Coils
--'07 BMW M5/6.:Indianapolis Red
'88 BMW 635CSi
'81 MB 500 SEC Koenig