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A flat battery means you can't get the deadlock or alarm system to release its grip
I'd start by charging the battery you should be able to do this via the cigarette lighter / accessory socket. You'll need the appropriate attachment for your charger but car accessory shops sell these- Alternatively if you can't get into the car/ boot the other way to do this is jack the car up so you can get under the front.
Locate the starter motor and the large power cable that runs to it - connect the positive battery charger lead to this terminal
then connect the charger negative lead to the metal of the engine block
This should complete the charging circuit and get some power into the battery allowing you into the car