should have back up capacitors, allow one or two unlock. if its dead beyond this you may need to contact dealer.
The computer key for the s class has an actual key incorporated in it allowing the car to be unlocked manually.
Replace the battery on your remote key. If you get the dealer to do it for you it is notoriously expensive. Did you try unlocking the passenger side with the key?
There should be a metal key hidden in the plastic key. It slides out of the side. i think this will unlock the door if the car or key battery are dead.
If the battery goes dead flat the door will still unlock manually with the key.
With your key.
Use the key to unlock it. The key is manually activated. Only the keyless entries will not work when the battery is dead.
Put the key in the door and turn.
Your battery is about dead .
dead battery
Key
The most common cause for a Mercedes-Benz not to start is a dead battery. Try charging the battery to see the automobile will start.
Use jump leads from another vehicle or charge your battery on the car
If you can not unlock them with a key, you will have to call a locksmith.