Hi, my name is***** your mechanic automotive expert, and I will be happy to assist you today. In some cases, I may need more information to assist better. I apologize for any delay and would like to know whom i am having the pleasure to chat with.
Dear customer, it could be that your Mercedes has run out of main battery or that the main battery of the vehicle is flatened maybe you could have left a light on at night or the battery has reached its end of its useful life.
We are going to start giving it energy with an external battery to try to operate and that at least the central locking system works.
To open the car, a solution you can take without having to call an auto locksmith is gaining access to the starter motor, to do this you need to take off the protection cover that is under the engine i,e under the car, you can lift the car or use a ramp.
Take off this cover and locate the starter, with jump wires and a auxiliary battery, connect the possitive wire to the starter postive terminal wire and to the possitive in the battery terminal.
And the negative to the negative terminal of the aux battery and to the other edge to any part of the chassis.
Then quickly you will push the key fob to open the car ( make sure the key fob battery is ok). This is the only way you can try without affecting any part of the car to open the car and replace the battery.
In case, you can do it, an auto locksmith garage or locksmith tech it is the best to sort out the problem, as they can help you open the car, the bonnet and reprogram or check the keys if it is needed.
There are other methods as gaining access through the grill to replace the battery and others on internet but i do not recommend most of them, because you take the risk of damage the lock or the paint of the car.
I hope this info to be helpful, my goal is to give you the best advice i can, I hope to be helpful, if you have any doubt or want to know anything else please do not hesitate to reply back.
Kind regards, Sebastian.