Have Mercedes Questions? Ask a Mechanic for Answers ASAP
How long has the key not been unlocking and locking the car?
I'm also assuming you have no spare any where?
I would say the key batteries could be dead if it was just the locking function that was inoperative but the key is designed to still start the car even with out any batteries. The batteries are just to send the radio signal for the remote unlocking/locking.
Does the dash and cluster bulbs light up when you turn the key? Does the steering wheel unlock?
Most likely it is just a faulty key then. The only way you can get a new one is to show up in person at a dealership with your registration and valid driver's license. Then the key has to be ordered. That is the policy in the U.S. at least. Canada might be a little different so I would call the dealership and ask them what their policy is on ordering keys.
Keys usually cost around $200-250 U.S. dollars.